Solved! Go to Solution.
I'm going throught exactly the same issue but my new modem IS listed under my account. Also using a new Netgear modem (CM1150V). I'm waiting on the powers that be to respond in my thread or for upper level tech support to give me a call back.
call 800-Comcast and select billing. ask the agent to update your gateway/modem on your account. have the HFC MAC Address available.