Connect it to the coax line and power it up. Let it fully sync up with the system ("Online" light is 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 account number 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 / HFC MAC address and the serial number of it handy when you call.
In that case you can try this;
Call 1-800-comcast and ask a rep to *roll and re-provision* the modem. Also ask them to confirm that the provisioning and account data in their database is correct and complete and that the correct configuration / bootfile for your subscribed to speed tier is being pushed to the modem.
YMMV. Good luck !
Amazing! I just got off the phone with Comcast and believe it or not what ever she did I am now getting 50 Mbps down and 12 Mbps up, we will see how long this lasts.