It seems that if nobody on your instance has previously interacted with a particular community from another instance, then it won’t show up under “all” in the communities list.

In this situation, you should be able to search for the full name of the community (!community@instance), that should trigger a sync of that community. After that, you’ll be able to subscribe.

Note that it’s also possible for instance admins to block some other instances completely - in that case, you won’t be able to subscribe to any communities from that particular instance.

Do you have the full link of that federated community? If yes, copy the entire url of that community, go to your /communities page and enter the url there and search. It will NOT show any results but don’t worry, what you just did is that you indexed that federated community for the entire instance.

Now delete the url from the search bar and search again by its name. You should be able to see it now and also subscribe to it.