Username and password required for Xfininty login

Hello. I just bought a Netgear AC750/EX3700 wifi range extender. Background: I have a place in the Colorado mountains. Xfinity/Comcast has put a wifi router on the utility pole by my place.  The signal in the bedroom is fine. The wifi signal in the living room is poor-to-nonexistant. Hence the need for the range extender. I have an Xfinity account that requires a username and password to login. When I try to install the extender is only prompts me for a password not a username. So my intall always fails. Am I doing something wrong or is my extender not set up for a username and password?

Thanks