This is partially a problem with how Lemmy works at the moment. Ideally, all communities should be synchronized and shown on all instances/servers, as of now any new severs from other instances will only show up on your own instance after the first time someone from your instance subscribes. This means someone has to manually type in the url and subscribe to an external community from your own server/instance so that your own instance can “discover” that the community exists and add it to the list. Hopefully this gets better down the road—I’m sure it will—but in the meantime https://lemmyverse.net/communities is a great place to discover new communities.