Activating Own Modem

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?