If it is a  First-time setup, we need to add the HFC MAC id either on the Xfinity App or provide it to Xfinity to activate it.

For the self-activation click the link below:

https://www.xfinity.com/support/articles/xfinity-app-activation.

Same problem.  Tried all the things: adding myself on the app, having Xfinity add, nothing works.  Always saying MAC ID is invalid.  What’s going on?

Have you logged into the modem to see if the mac in the software compares with the mac on the device? 

I’ve had one with sticker over it before

I have not!  Great idea!  Will do so.  Thanks!