I wish I had another wifi router to check against besides my phone. The Xfinity router part number is CGM4981COM or XB8. Phone is a Samsung S20 G5. nixOS 24.11 fresh install no modification.
Finally figured out the easiest solution to the problem. In the Xfinity phone app you can select the version or Wi-Fi a, g, n … for 2.4 and 5Ghz, so I turned off the ax on both and it connected. Hope this helps someone else. AI and Xfinity didn’t.