activate xb8 gateway using laptop

Connect it to the coax line / outlet and power it up. Let it fully sync up with the system. Check that the front or the top panel indicator light(s) is / are steadily lit. 

Then 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 self-install 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 activated / provisioned for service. Have the RF / CM / HFC MAC address and the serial number of it handy when you call.

P.S. Disconnect the old gateway first.

Using you phone, call the xfinity activation line at 1-800-931-0790.

You will need to have the modem nearby, as you will have to read some numbers from it's label to the technician that activates it.  The modem will need to be plugged in to power and attached to your cable. 

@NoNoBadPuppy 

Please let me ask you. Was there anything wrong with the self-activation number that I provided? It's the one that's worked for many years, and still does..