Why can't I subscribe to communities from other instances federated with lemm.ee?

As the title sais. I've just created my account on lemm.ee and wanted to subscribe to some communities that I used to follow on Reddit.

For example, I've seem that there is ABoringDystopia on mander.xyz, so I tried looking for it using the search tool in the communities page, but nothing showed. I then tried accessing it using the url https://lemm.ee/c/aboringdystopia@mander.xyz, but all I got was the error 404: couldnt_find_community.

I've had similar issues with other communities too, from other instances.

The problem only affects some instances because I've been able to access communities on feddit.de, for example.

Can someone more navigated tell me what's going on?

P.S. mander.xyz is even in the list of instances federated with lemm.ee, so I should be able to access its communities.