You can easily moderate communities on remote instances, all you need to do is leave a comment there, and then the existing moderator has a "make moderator" in the context menu below your username.
This can't currently be done through any app, only in the web front-end.
In case the community is de facto unmoderated, you'd have to contact the admins of the instance. Or alternatively, create a community with the same name on your home instance and leave a comment in that community informing them and hope some will follow.