I recently got a Google Pixel 2 and switched from an iPhone 7 where connecting to the XFINITY secure wifi was seamless and a certificate was installed on the phone. I work as a network admin so I know a thing or two about wifi. So the issue with the Android, is when connecting to the secure XFINITY wifi, it prompts me for EAP credentials (802.1x authentication, identity, and certificate are all requested). I just finished a chat with support who escalated this issue because installing the xfinity wifi app on Android and signing in only works with the unsecured xfinity wifi.
I am most curious of when Android users will be able to join the 21st century and "allowed" to use the secure XFINITY wifi that we are all paying for.
Have you tried uninstalling reinstalling, works fine for me on my cheapo LG. I have it set to auto connect and it will connect to both Xfinity and xfinitywifi as I travel around town.