To test I just added Xfinity-TVE, no problem.
DVR version 2021.11.23.2219
I already have 2 servers with it working, so added it to my docker container that had only a Pluto source.
Then enabled local networks and got them also.
Not sure what's different for those unable to add it.
I'm on my home Xfinity network using my own account and used a secondary VIEWER only user/pass.