I follow mma and see they have a community on kbin. Can I somehow subscribe to it from lemmy.one?
Yes, just copypaste the kbin community link on the searchbar of your instance (in your case it should be https://lemmy.one/) and then you can treat it as if it were a lemmy community.
I did just that but there are no results. I used this link:
Same thing on desktop.
EDIT: @mma@kbin.social tried this one also but no luck.
Try this link: https://lemmy.one/c/mma@kbin.social
That did it, now I’m subscribed but I only see few posts and users (1 subscribed) on desktop (Lemmy) and Jerboa but when I go directly to kbin on desktop I can see much more posts and users (253 subscribed).
You only start getting posts and comments from after the first person on your instance (you in this case) subscribes to the community. You can make it fetch old posts manually if there’s something you want to reply to by entering the post URL in /search, otherwise just wait and all the new posts will show up here.
