Yes, you can rent their modem or buy your own.
We did originally have a modem issued by them. We had one for cable internet and TV. When we downgraded to a basic package, we no longer needed the multi-service modem and they swapped it out for an internet-only modem.
We began having major problems with the walljacks, which they insisted was the modem. They wanted me to wait a week for them to come replace it, and they said if it turned out to NOT be the modem they’d make me pay a housecall fee. So I went and bought my own new modem.
And still had the same problem, so it wasn’t the modem. I was able to get the internet to function by changing which wall jack it was hooked up to, since they were insisting that it was my problem and if I couldn’t fix it they’d have to charge me to come do so.
Then that jack failed. It worked, and then suddenly it did not work anymore. Then, a third jack failed, in the same way. We are currently using the last of four that we have in the apartment, and if this one fails we will have to pay them to come back out and look at the connectivity again.
Every time a jack has failed, I have noticed a Comcast van outside. I don’t know if them working on something else is interfering with the lines for my apartment, or if there was some kind of problem elsewhere they were called in for, but it has happened that way every time. The last time our building had an outage, the tech that came out disconnected all but the current jack we are using in order to get it to work at all. He tried the others and couldn’t get them to work, either.
Long story short, instead of returning my own modem, I just gave them back theirs, since we were paying a rental fee on it anyway at the time, and buying our own eliminated that fee.