You can build your own APK of Infinity with your own API key. You don't need to install any apps (other than infinity) and it can be done from your phone.
Screenshot in case the post on Reddit is removed:
Edit: Also, Geddit (Website) (F-Droid) (GitHub) works without the API, but you can't interact with anything, comments don't show properly, and I think the UI doesn't look very good.
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 https://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.