don't know if you found the answer, but in case you havent:
This is for if you are on desktop. Don't know about mobile apps. warning: It's a convoluted process.
- go to browse.feddit.de: https://browse.feddit.de/
- type in keyword (eg, "factorio")
- copy the link from the lemmy community (the green lettering or the copy icon beside the green lettering)
- go back to your instance
- hit the magnifying glass by/near your profile
- paste the link in the "search" field (make sure you are searching "all")
- click the link that comes up, which will take to you the lemmy instance
- hit "subscribe" once you are there.