Not sure if you got it sorted or not, but if you were following the docker-compose method documented by the devs, there were a couple hurdles I ran into. The one that may be relevant here is that at some point their docker-compose.yml did not expose the Lemmy backend to the Internet, and so all federation was failing. That said, I checked just now and they seem to have fixed that issue upstream. So you should be able to re-pull their docker-compose.yml and it should work.