Have you tried uninstalling/installing it (clearing cache and data)? Could be it somehow got out-of-sync with your account and needs to be reset.
Thanks for letting us know and apologies the app is not working as it should! Could you try uninstalling/re-installing the app using the steps below:
Go into settings > apps > Xfinity MyAccount app (may need to tap "Storage" to get to next step)
Clear app cache and data.
Once cleared, uninstall the app.
Reboot your device after you uninstall the app.
Once the device has booted back up reinstall the app.
Try to log in and let us know if you still have the same issue.
I have the same problem and have tried all the suggested remedies to no avail.
Are you able to connect to it with a computer?