Remote access of other communities is a smoldering mess. You have to hope the community you want to subscribe to shows up in All or you have to know its name and use the search feature. There’s no way to see another instance’s communities, unless you have an account there, which is a huge problem because you have to go through the huge hassle of signing up for an instance just so you can find the community, and oh yeah, most instances aren’t even open to invitation so it’s not even possible in a lot of cases.
Also if you get banned from a community it force-unsubscribes you, and if you get banned from an instance it won’t even tell you.