I've had issues doing it from mobile, but on desktop this is what I do:

• Search the community on this Lemmy Community Browser
• Copy the link (e.g. https://ttrpg.network/c/dungeonworld)
• Come to lemmy.one, click search
• Make sure everything is set to All (type, sub/local/all, community, creator)
• Search the link in the box (https://ttrpg.network/c/dungeonworld)

It sometimes takes a bit to search, but eventually it will and will present you with a link titled dungeonworld@ttrpg.network with a destination of https://lemmy.one/c/dungeonworld@ttrpg.network

And voila. If a community hasn't been subscribed to from your home instance it's my understanding that this is the only reliable way to get it federated. Once you do this search it will show up but no comments or anything come across until someone (e.g. you) actively subscribes to the community.

I did this for dungeonworld so you should see something now, but give it a go for bassment.