Same problem. The "Xfinity Assistant" sends me around in circles. I just want to add the new MAC to my account.
When I started the service, I spoke with a human to add the MAC. I can't find that phone number!
I need to talk to a technician to setup my new modem but I can't find a phone number
Connect it to the coax line and power it up. Let it fully sync up with the system ("DS, "US", and the "Online" lights are steadily lit). Connect a computer to it and open up a browser. You *may* see their "Walled Garden" modem self registration page. If so, follow the screen prompts. Have your mobile telephone number, your Xfinity username and password or the account number and telephone number associated with your Comcast account handy.
If not, try calling 1-855-652-3446. That is their device activation line.
If that doesn't work for you, you can call 1-800-comcast and tell a rep that you would like to have a new modem provisioned for service. Have the RF / CM / HFC MAC address and the serial number of it handy when you call.