Early on when I first joined lemmy.world there were communities from other instances that I wanted to subscribe to but they didn't show up when I searched. I read somewhere that you could paste the external community's URL into the search bar and then a minute or so later, the community would appear. I tried this and it worked. It's as if searching the URL forces your local instance to reach out and find that community on the other instance. Maybe this can help in your situation?