You have to find the same community in your home instance, instead of going to that instance directly.
I agree this is confusing, especially if you come there from a link. There’s no easy “go to this community on my instance” link right now, though there are user scripts to make this easier.
What you can do is copy a link to a community or post and paste it in the search on your home instance. That’ll hopefully show that post and allow you to subscribe.
You have to find the same community in your home instance, instead of going to that instance directly.
I agree this is confusing, especially if you come there from a link. There’s no easy “go to this community on my instance” link right now, though there are user scripts to make this easier.
What you can do is copy a link to a community or post and paste it in the search on your home instance. That’ll hopefully show that post and allow you to subscribe.