When you go into the community search on your instance, make sure you select All instead of Local. You’ll get results from all of the instances yours is federated with then.
The community catalogs people post will often take you straight to a different instance—which is different from the one you made your account on and logged into—which is why you see a login prompt.
Searching from your instance with All selected will let you visit communities from any instances yours is connected with while you stay logged in on your home instance. Hope that helps, I tried to simply but it’s late and I’m tired.