You're supposed to use one of the methods described here, to access outside communities from inside this instance https://tech.michaelaltfield.net/2023/06/11/lemmy-migration-find-subreddits-communities/ There are more guides in the pinned post here.

But... (tl;dr) earlier today I got only one method to work, and now it gives 404. Stuff seems to be breaking due to traffic (?). I'll try to ask under the guides post. What worked earlier was going to lemmy.fmhy.ml/c/@, where it might be e.g. = warframe, = dormi.zone in your case. You get the community name by going to its page (on the original server) and then it will be below the title after !, like !warframe.

The community tab here is supposed to show you all federated communities in "All" once at least one person subscribed to them so this should be easier.