Looking at your Nginx conf, it seemed that we actually have almost the same configuration so my problem should be coming from somewhere else.
The fix is actually quite simple and I feel dumb not to think of it sooner. I simply needed to open port 32400 (Plex default) on my firewall, which is dropping all connections by default (I’m using iptables with DROP policy).
Then using Plex default port in Settings > Remote Access and my server is now visible from any Plex apps! 
