For the community to show up on an instance, it needs to have been viewed by at least one person from that instance. (You need to be logged in for this to count).
I went to lemmy.one/c/[email protected] with my alt on that instance. Now you should be able to see it from there, even if you aren’t logged in.
Also, I think for posts to be federated properly there needs to be at least one subscriber from the instance (just visiting isn’t enough). I’m not sure about that tho.