When you’re searching communities, it only shows you communities that are hosted on your home instance or that someone on your instance has either searched for before or subscribed to. So, if you know the exact link you’ll want to search for that, then once you do it takes a few minutes to federate if nobody has searched for it before. Give it a few, search again and it will now be in the results so you can subscribe. Otherwise, it might be helpful to have an account on a few high traffic instances so you can search from there to get better results and get the link to take back to the instance you want to actually use it on. I haven’t tried using a search engine to find communities but that might be another way to find them.