Did you check if it's defederated? Would be the first thing I do, that instance is a bit controversial.
In a browser, go to yourinstance.com/instances and navigate to "blocked instances" on the top.
Second thing to try is just link it here in the comments, with a leading !, such as !germany@lemmy.org - you should be able to follow the link in app and then subscribe from there.
Lastly, if it's a very new community, it's just a waiting game until your instance has synced up.