You need to file an FCC complaint about this, Xfinity has an executive response team for all FCC complaints. Document every time you called them and your equipment details.
I had to do this when they let a new renter cancelled my cable internet multiple weeks before my move-out, even after I told them that the new renter's dates were wrong when they called to confirm. Regular support wanted me to sign a new contract and pay an install fee to reactivate.
It was a really shitty situation that should never have happened, but the executive team was what all customer service should strive to be. A single person, with ample time in their day to fully understand the problem and blow past all the usual roadblocks on the way to a solution. Was still a few hours on the phone, but at least I wasn't treated as though it was my fault.
Honestly, that experience was years ago, and I don't really care enough to raise a fuss about it just today, since their first-party router is "not too terrible".
About Xfinity in particular though, assuming someone else uses them, I did learn just recently that at least Xfinity allegedly offers a special decoder box (or something) that is, I believe, free of charge, and lets you hook up your router directly rather than putting it through theirs as a DMZ, and it's supposed to get them to not cut the line. Some self-install kit or something. You can only get it by asking over phone call.
1. You had a modem that wasn't on their approved device list. It needs to be on this list to receive firmware customized to their network parameters. https://www.xfinity.com/support/devices/
2. (the most likely scenario) You were switching the macaddr of your router's WAN port every time you connected. For IPV4 Comcast was always set up to only allow one macaddr to bind to a public address at a time, you needed to trigger a dhcpcd release on the old macaddr before reconnecting or clone the mac of the old device if switching.
I didn't have to file a FCC complaint with Xfinity but I had an issue with Arris/Moto surfboard modems and a miswritten boot file some years ago that took weeks to fix. Basically, you can have your own equipment on a dynamic IP'd business account but they automatically default to calling you the asshole for any problems that get reported.
Business class had launched new speed tiers and it broke upstream bonding on my modem, of course they blamed me. So I bought 2 more modems (same model and a different one) and the same things happened as soon as they got provisioned on the account (prior to this on Comcast's provisioning walled garden they'd show the upstream as bonded, but once activated on the account upstream bonding would turn off and speeds went to shit). By this point I had records and emails with like 4 or 5 different people, a useful thread on dslreports, and a very worthless thread on reddit.
I took all this info and CC'd everyone I spoke with in an email to the CEO of Comcast. Executive response had someone up in Massachusetts call me the next day confirming the issue was with their provisioning boot files, and they credited me for like 3 or 4 months of service. Overall I was pissed it was this difficult to escalate but at least once it was in the right office they handled it. It probably helped that this was a business class (which overpays for better support vs residential script readers).
