Activate new cable modem

I purchased a new 3rd party modem. Xfinity directs me to activate modem by using the Xfinity App to register the MAC address of the modem. This registration process keeps failing. I attempted to call the 1-800 support line to manually register MAC address with a live representative. The support line directs me to download and register the modem through the app, then the call disconnects.

Can someone on the Comcast support team please reach out so that we can manually register this modem? The registration process through the app is not working.