You can subscribe to any content anywhere as long as neither instance is on the other’s block list. If your instance hasn’t federated with the other instance yet, then you need to do that thing where you search for “!community@instance” and wait a few minutes. Otherwise, you just go to that community while on your home instance (e.g. for you, “mander.xyz/c/community@instance”) and click subscribe.
TIL. I guess they must have improved it recently. That’s one of the worst parts about starting on a smaller instance for sure. Being able to copy+paste the URL from its source is really helpful.