Replacement modem activation failed

I was trying to get a replacement third party data/voice modem activated today. It's one of the models recommended by Comcast (Netgear Nighthawk CM2050V). Modem booted without problems and internet indicator was steady after a few minutes. The self-activation process did not work because I was unable to access the activation page listed in the Quick Guide. Since my phone service was out with the old modem being disconnected, I reached out to Support via Xfinity chat (from a computer connected to a neighbor's WiFi). Provided the usual info (CM MAC address, modem model no etc) but none of the 4 agents that communicated in the chat were able to successfully activate the new modem - they all claimed it was activated but neither internet nor phone worked. For now I am back to the old modem which immediately worked fine again. Any suggestions what else I can do to get the new modem activated?