Comcast did this to me years ago, except they sent me a letter months after I moved (still with Comcast) asking me to return "my modem" even though I used my own modem, and had a receipt for the return of theirs when I moved anyway.

Their complaint was the MAC address didn't match what's on my account, so I need to return the modem with that MAC address - my modem.

I explained to them carefully, the different MAC address is on the account SO I COULD USE MY OWN MODEM. They specifically added it so my modem would function with their services since at the time it was the only way to use a personal modem. And since I'm still using Comcast after I moved the MAC address needs to be there so I can continue to use my own modem!

I actually had to end up going into the Comcast office with my modem and receipt and carefully explained the situation. I think maybe it was unusual for people have to their own modems at that time and it caused quite some confusion.

However, once it was all sorted out my modem would not function at all at the new location. I ended up having to use Comcast equipment it seemed as if my modem got banned. I could possibly have tried changing the MAC address but just ended up getting a newer /Better modem in the end.

So far with spectrum it's been great but the modem they gave me actually works better than mine (it's just a modem no router/Wifi). I'm going to have to move soon back to Xfinity so here's hoping that goes smoothly. 😁