Logging into Xfinity Router

I am unable to log into my home Comcast router. The user name and password I recorded does not work. How can I resolve this issue? Do I need to reset the router to factory standards and then rebuild the work that was done in the router? Is there a way to retrieve my user name and password?

I have a Sonicwall router behind the Xfinity Router which I use for VPN access. I have created a DMZ and excluded that IP from the available IP addresses. I have also changed the sub net from 0 to 99 (10.0.99.XXX). This worked for about 2 week then suddenly stopped. I need to access the Xfinity Router to troubleshoot. But I can't log in.

I have contacted support but they just told me to use the xFi app and that is of no use for my issue.

Any advice would be greatly appreciated.

Thanks,

pat