hey @Jattatak, you seem to be the only other person I can find who is facing similar troubles to myself when trying to set up a lemmy instance. I've redone my docker-compose, nginx.conf, and lemmy.hjson to be exactly the same as yours (with some changes in the password / domain name). I'm also running an nginx proxy manager container.
However, it seems I'm still having the same problem of being able to see post content but not comments in other instances. I have the added problem of when trying to post a comment on my instance, the form freezes until I refresh the page. The comment does actually get posted.
I've also made sure the 'lemmyinternal' network is not isolated. I wonder did you manage to do anything to trouble shoot this issue? are there any ports I need to open on my firewall beyond 80, 443?