Originally I had a lemmy.ml account but I decided to make a new account on a new instance. The problem is that on sopuli.xyz, I can't find some existing communities.
What is the reason for this?
Posting until someone comes along with a more in depth answer but essentially your instance won't link to others until a user has searched for it. If you joined a smaller community chances are no one from sopuli is currently subscribed to that community.
I'm having trouble doing this on mobile (probably given the slowdowns) but you're supposed to be able to search using the !community@instsance string and it will pop up after a little bit. Add it and future users from sopuli will see it in all.