When I replaced the Xfinity modem with my own yesterday, the change was automatically detected and I was walked through activation of the modem.
The modem worked fine, but I had some issue with Wifi, so I swapped back to the Xfinity modem.
After researching the Wifi issue, I tried it again today, but no such walk-through happened.
How do I activate the new modem if I'm not getting any internet connection?
Give this automated device activation line a shot: 1-855-652-3446. Even if the automation fails, you then get connected to an advanced repair representative rather than a tier 1 phone rep. Good luck !