What did @hotmail.com tell you?

I have a question that might be right to ask here, idk.

My account is on lemmy.world.

Ive added lemmy.ml and a few other instances.

I have subscribed to a few communities on lemmy.world, but when i try to do the same for communities on other instances I've added, it says i need to be logged in to that instance. I thought part of the way this worked was only needing one account to interact with all federated instances? Can somebody help me wrap my head around this? Strictly mobile user