You don’t log into the other instances to access their communities. Instead, you log into your local instance and use the search function to search for the target community. Just enter the community name like that: !nameofcommunity@nameofinstance.
Alternatively you can click on Communities at the top (or the equivalent in your app), select “all” and browse all the communities anyone on your instance has ever searched for using the first method.
You don’t log into the other instances to access their communities. Instead, you log into your local instance and use the search function to search for the target community. Just enter the community name like that: !nameofcommunity@nameofinstance.
Alternatively you can click on Communities at the top (or the equivalent in your app), select “all” and browse all the communities anyone on your instance has ever searched for using the first method.