I have not seen this for a long time, though its something quite familiar:
Some minutes after starting the app this nice little error message pops up:
iOS 14.6 updated today
Restarting iPhone 11 did not solve the issue.
Last time I have seen the app working as expected: Was in an Alpha-battle and the phone lost connection - I was 2,5 m away from my router without a wall in between.
Connection to the internet with other apps work as expected.
For some reason iPad - iPadOS 14.6 - does connect with no issue at all, at least it does currently. Same local network.