Currently searching for existing communities on other instances doesn’t always show results (if no user on your instance has already subscribed).
A workaround I have found is to search for a post from that community, after which you can access the community via the link near the username of the OP.
From there you can subscribe.
That’s odd, it works perfectly for me, that’s even what gave me the idea of a script to retrieve all the communities from an instance.
yeah, it may have been impatience on my part. Great script idea, btw!