I’ve seen this mentioned a few times already: people wondering why certain communities don’t show up in their instance. You need to first search for that community via the magnifying glass icon and type in “!communityname@instance.domain”
Notice the exclamation mark at the beginning. Might take a few seconds. This will teach your instance about the remote community.
Copy the URL and paste it into the search bar of your instance. That will force your instance to “Discover” it. It might say no results but go back to the community page then search the name again