An external community needs 1 subscriber to be visible in ttrpg.network, after the first subscriber it's visible for everyone.
This is a bit more complicated than just searching for the community in the list of communities.
Here's what works for both lemmy and kbin.
Put the url of the community in search:
- lemmy example "https://lemmy.world/c/lemmyworld"
- kbin example "https://kbin.social/m/science"
click search, click the result, then subscribe.
After that it's visible here https://ttrpg.network/communities/listing_type/All/page/1
And for example science is here : https://ttrpg.network/c/science@kbin.social