Switching devices solved it for me. I’ve tried your steps on Android (both in Jerboa and Chromium) without success. But on desktop it worked somehow and now I’m successfully subscribed to both communities. Thanks midnight!

Do you have an idea why specific communities of federated instances don’t show up, e.g. !dungeonworld@ttrpg.network or !bassment@feddit.de ? When I search for them (either !x@y or https://y/c/x) I get no results, even after a few days of waiting.