I have been using Plex on an Arm based Synology for many years now and it is configured for remote access. I have a Let’s Encrypt Certificate on Synology. I ran into some issues with the server not being powerful enough to transcode.
I have also recently added a Windows 10 mini PC and installed Plex on it. I can locally access movies without issue and I have enabled remote access. I added the Port Forwarding rule to my FIOS Router and I can access it remotely but it has not been reliable (it often shows as offline) and it is also showing it is being relayed. I suspect that not having a Certificate installed on the Windows Mini PC may contribute to the connectivity problem but it is not apparent where to start to address the problem.