Checked that app on mine just now and it loaded normally. My first guess was a bad version of the app.
It appears you've tried the things you can try as a user. It's peculiar you are having the same problem on TWO AppleTVs. That likely rules out a bad app install or something like that.
- Reboot your router?
- Reboot your modem?
- Manually close all open apps on the AppleTV and then reboot to be sure that RAM is as free as possible?
- Xfinity working right now? Check their website and they can usually identify any outages of very localized service?
- You have an active Xfinity television subscription plan in effect right now? It needs active television service to work. If you were perhaps leaning on Peacock (only), I think I saw something about them stripping that out as a separate service within the last few days. Do you get any Xfinity television signals on a household TV?
- Try the website version of the same? See if you can access television through the Xfinity website? If so, the app should work too.