It should be the hfc mac that they use. 

Are you activating it through the xfinity app? 

I tried the HFC MAC number and successfully activated my modem thru the Xfinity app this morning. Thank you for the suggestion.

I spoke to Xfinity customer service for far too long last night. I gave them both the WAN and HFC numbers. They said I was activated but could not explain why me trying to login into the modem only resulted in a redirection to the Xfinity app to activate the modem. They concluded it was an Arris issue by not providing a CM MAC number. It seems they, like me, did not know CM MAC and HFC MAC are the same?

Thanks again.

Glad you got it working!

the app tends to be better/smoother versus calling them