For a remote community to show up on all, someone on the instance has to be subbed to it.
So, if you type !community@server.com in the search you'll see it, and be able to sub as the first person on your instance. Once done then it'll show up on all to other users.
Sometimes takes a few seconds after you hit "search" for it to pop up. Has to fetch content and process it, etc so give it about ten seconds and it'll be done.
Exception to this: if your instance blocks server.com, then you won't be able to sub to communities on it. Most instances don't block much though