Okay, posting as a Former T-Mobile Tech Support agent (now quite a few years ago).
@PastTense has done good work on the details, so yes, I wouldn’t advise adding another line with Tello unless 5GB per month covers the usage fully. And streaming, depending on the quality, is one of the big data hogs. Assuming 720P, that’s roughly 5 hours of video (lots of caveats). Higher quality is more, lower is less (of course).
You’d also want to use T-Mobile’s coverage map on their website to check - it could be good, average, or poor, which will determine what sort of speeds they get, and of course, if the beater phone supports 5G and the other locally prominent bands.
Another thing - as a reseller, say, 95+% of the time, it’s not an issue. That last 5%… if there’s an outage, congestion, or other event in the area effecting coverage, well, the resellers get low priority, which is stated in their contracts. Again, -rarely- an issue, but it happens.
As for making it work - I have done this on a spare line, which had a 10G cap for data. I basically took the spare phone, turned off auto-updates for the OS and Apps, put it on a newly created Android Account NOT linked to my normal one (so if the phone went… astray… it didn’t have any of my personal info on it) and spent some time in the settings to make sure the Wifi hotspot went on and stayed on.
Then left it plugged in on a shelf at my father’s workshop so he’d have wifi for his security camera (long story). It worked… okay. If there was a power failure, he was good enough to be able to restart the phone and about half the time, it would work without needing anything else. But the other half of the time, I’d have to go fiddle with it because something hadn’t come back up properly.
So, you known your mom better than we do. Is she comfy with these things? If not, is she good at following instructions if you walk her through it on another line (no judgement, my in-laws are pretty good at this, my mother and father are TERRIBLE at it, thank god for TeamViewer)?
And how does your mom view the NFHS? On a phone, a tablet, a Smart TV or stick on a Dumb TV? Does she normally use a wireless connection or ethernet connection for her DSL? All these things matter especially if she’s not comfy with the tech.
In that case, then yeah, if the T-Mobile (other carriers are offering similar service depending on area as well) 5G internet is available in her area, it provides ethernet and wifi service in a single device. It may be the easiest set up, and while I’m no longer part of their tech team, she’ll at least be able to get help with it, which will likely NOT be case if she’s trying to just use mobile hot spot.