I finally managed to get DNS working on my server, which means I could update the certificate for the misskey instance!
And then I could fetch a certificate for THIS instance - after figuring out that the gotosocial systemd service failing was due to the directory /var/log/gotosocial not exisisting, so it couldn't write the log.
There seems to be some federation issues currently - I don't think follows are going through / being resolved properly. Not sure how to debug, but it'll have to wait in any case.
Now it looks like it's all working. At least I can follow and be followed by my other accounts.
Didn't actually do anything though, so I don't know what happened before.