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 “[email protected]”
Notice the exclamation mark at the beginning. Might take a few seconds. This will teach your instance about the remote community.
For real. I feel like I recall a browser extension doing something similar for Mastodon. But I don’t have the coding knowledge to adapt that to this use case.