You're in fmhy, which is not federated to some few instances (Lemmygrad is one I know, for instance). There's a map somewhere, I'll link it here later, so you can check if an instance is federated to another. If they are, users from both can interact with each other and subscribe to each other's communities.

So, a few concepts first: to subscribe to a community in another instance, SOMEONE from your community must have subscribed to it before you. If someone did, you will find it on Jerboa (search, second button on bottom, from left to right). Use the feddit link providrd to find a community you like, no need to copy the link to it. Just type the name and it will show up.

Now what to do if no one has subscribed to it yet? You can be the first to provide it to your entire instance, BUT not from Jerboa (yet). You gotta login to your instance via web browser, go to communities and search for the full link (two formats: [email protected] or instance.xyz/c/community). It will return no results at first, but it will fetch and then you search for the community. Sometimes, you gotta refresh and just search for community again.

That's it. Remember: it will only work if the instance you're logged into and the one that has the community you want to join are federated.