This says ZTE corp. (a Chinese company zte.com.cn) which doesn’t help much. They make assorted products.
Sorry for the delay. Echo answered the question for me, and the other responses are helpful as well. So if i am looking for something like this on my network, it is not quite so easy. I currently have 4 wifi access devices split into 8 different points. There are 10 people, each with a phone, a computer, and there are also several tablets and gaming consoles and TVs or such devices. My computer has up to 4 additional virtual machines, and there is 1 other server that has up to 5 vms. At one point, one of the kids downloaded something, got a virus and i had to identify what was going on. Best for me was to shut everthing down, reset each router and start over. So as the devices got reconnected i was able to name any that didnt have a name, and I was able to setup a bunch of reservations as well. It can be quite the task. But as others have said, turn off all the devices, reconnect 1 at a time, the you can see.
Now there’s a blast from the past.
I think I know what you mean, but that description as it stands isn’t accurate. The MAC address is only relevant in a Local Area Network (usually Ethernet or a broadband connection to a cable operators’s CMTS or equivalent copper or fiber DSL) in most cases solely for requesting an IP address from the DHCP server. To do useful application-level communications either within a LAN or in a WAN, at a minimum you need a network protocol (typically the “IP” part of “TCP/IP”) and a transport-level protocol which deals with end-to-end transmission. TCP is one such protocol, and provides what’s called a connection-oriented path, meaning it works like a virtual fixed connection, routing around failures and providing for packet re-transmission in case of errors. A simpler alternative sometimes used for some applications is UDP, which is a datagram protocol – a datagram is a packet sent into the network on a wing and a prayer and the best wishes of the sending device, but it may be lost or corrupted with no recourse.
I think what you mean is that with dynamic IP address assignment, the MAC address is usually or often the only constant ID associated with a LAN device. But as previously noted, even that isn’t true for devices that generate virtual MAC addresses.
Isn’t it? 
One of my supermarket shopping bags has Digital Networking emblazoned on the side. I can’t remember where it came from. A trade show somewhere I suspect.
I’m pretty sure I have an original DEUNA hexcard pair somewhere. That and a vampire adaptor. From memory, even the DEUNA allowed for software setting of MAC address.
A blast from so far in the past that in the context of MAC and IP addresses, I just vaguely remember that DECnet adapters in conjunction with the DECnet protocol provided for a direct mapping between DECnet addresses and MAC addresses, which was clever but proprietary to DECnet.
P.S.- DECnet existed well before the VAX, though I don’t really remember how long before. It’s all just a dim blur now! 
Having played with thickwire and vampire taps (for transceivers) to set up DEC ethernet helped me answer the trick question on the Microsoft neworking test about wiring - so I got 1000/1000 on that exam.
There was a way to change the MAC address in software for those 3C509 cards to work as clients on DECnet… DECnet software did it as part of startup, but you could do it yourself as a utility if you were going to try spoofing.
As I understand it, MAC is the default for ethernet to ethernet (wired or wifi) protocols - or at least, wired. A packet from one ethernet port is addressed to another ethernet port. If the destination is not on the local ethernet, it is sent to the MAC of the default gateway, unless the IP knows adifferent gateway to use.
DOS command “arp -a” shows the MAC to IP translation. your computer is aware of.
To see if a device is alive on the network, figure out its IP addres.
Then delete the ARP cache entry. “arp -d x.x.x.x” where x’s are the ip address.
ping the IP address. “ping x.x.x.x”
The first thing your PC does is use an arp announcement to ask “who is x.x.x.x”?
The device will respond “I’m at MAC yy:yy:yy:yy:yy:yy”
Now the current MAC for that IP is in the ARP table.
Now your device will send ping packets to that MAC.
I did mean that the MAC Address is the unique identifier. It is what identifies a unique piece of equipment on the network. Most Network cards have the MAC set at the time they are manufactured. However, many devices can “create” their own MAC address. Yes you need an IP address for most networking these days, although there are still other types of networks in use that do not use them. But without the MAC address, you would not even be able to get an IP address from a DHCP server. The MAC address is how data is routed on the Network you are connected to, and the IP Address is how data is routed throughout the internet. This is very simplistic since the IP Address you have on your computer right now is most likely not Internet routable, meaning only the main network router will manage traffic in your local network. If you look at your IP address now, it most likely starts with 192.168, or 10.0. Those addresses are not routable to the Internet at large and must be hidden behind a router. Only your router and other devices on your network would use those IP addresses. In addition, if you run ipconfig /all from the command prompt in windows, you will most likely see that your card has 1 MAC address (or physical address) and 2 IP Addresses. 1 that is IP v4 (something like 192.168.0.11) and one that is IP v6, which is a much longer string of numbers and letters.
So all of this is more than likely much more technical that the OP wanted to get into. So OP, hope you found the device causing the issue. If not, let us know, I am sure there are other ways some of us can help.
DECNet was created for the PDP-11. I used to know how to configure it on the RSX-11 operating system, but that too is just a dim blur. Pretty much everything I used to know about RSX-11 is a dim blur. @[1,1]shutup (shutdown the computer) is the only thing I really remember these days.
Both wired and wifi.
Alas, I have had no luck. I disconnected all wifi capable devices that I’m aware of (and thankfully I have a much simpler collection of devices than SoToasty). Restarted the router and the unknown device was connected. Does that mean that unknown device is the router? If so, why would my wifi notify me of a new device having been connected?
How many devices do you see connected? Two? I’m assuming you can see the computer/phone/tablet you are using to access the router. Is the unknown device a different one?
Correct there were two - unknown and my laptop.
It means it is a device you are not aware of, which is sort of a tautology. That does limit the search to things you didn’t think about on the first pass. Depending on your router, it might also show wired devices. Do you have anything connected wired?
One that has confused me in the past is my watch. Usually it gets networking through a bluetooth connection to my phone, but sometimes it decides to connect to the wifi. The watch also randomizes it’s MAC address, so I was seeing intermittent connections from an unknown source, and it took some time to figure it out.
As suggested above, can you block the unknown device via MAC address?
Of course, it might just create a new one and re-attach to your network.
Just grasping at straws here…
Silly question (might have been answered already, if so, ignore)…is your laptop plugged into an Ethernet cable? If so, you could have a wireless connection and a wired connection at the same time from the same machine. I have done this multiple times and scratched my head a bit until I realized that it was the wireless connection, with its own MAC address, doing the other connection.
Laptop was not plugged in with an Ethernet cable (I’m not sure I even have one!).
And I don’t have anything like a smart watch or an Alexa. This is everything I can think of that could connect to the internet (starred items are or were recently connected):
*TV
*Laptop
*iPhone
*Dyson air purifier
*Washing machine
Dryer (not connected. I cut power to the laundry room and the washing machine has not reconnected, so I think this rules the dryer out as the unknown device)
Kindle (identified but not connected)
*Ring alarm base
*Ring camera
Car
Printer,
Game machine like PS4
Apple Air Tag
Refrigerator
Thermostat?
That is about all I can think of right now.
Nope none of the above. Not even a smart toothbrush (which is apparently a thing now?)
Just brainstorming here, but it seems to me that most simple IOT devices like washing machines and toothbrushes will use a static MAC address with a normal manufacturer-issued OUI. Since it seems that the mystery devices is using a dynamic MAC address, it’s more likely to be a sophisticated device like a phone or computer.
Worst case is that a neighbor has somehow connected to your network, which isn’t terrible if all your devices are locked down, but a lot of networks are not configured that way and depend on the network itself being inaccessible to outsiders. (Your router uses WPA2 or WPA3 with a strong password, right? If it’s using WPA or WEP you should change that immediately.) If possible, I would try blocking the MAC address as suggested earlier.