We are experiencing the same issues, as are many other Xfinity customers (according to a Google search and these forums).
We have a 2019 Samsung Q70R smart TV, and have been Comcast customers for over a decade. I am a little surprised that Comcast/Xfinity has an app in production that is a "beta" version after so long in production.
The Xfinity app works fine on my iPad and iPhone.
That said, I have tried all of the aforementioned fixes with negative results.
-delete the Xfinity Stream beta app
-check for TV software/firmware updates
-unplug the TV for 3-5 minutes
-plug back in and turn on
-download and reinstall the Xinity app
-try to log in
-when I do so, and obtain a access code on my tablet or phone (on my Comcast/Xtinity home network)
-I input the access code on my device and see the "success" green checkmark
-the TV screen changes and moves to a new screen - Welcome! This device can only access Xfinity Stream on your in-home Xfinity network. Use the app on your smartphone, tablet, or laptop to watch on the go, anytime you like.
....it worked in the past,????
A frustrated paying customer.