Both, thanks for the responses.
As to GDM ports being blocked, I don’t think that applies as I’m not aware of a firewall on my Synology NAS being active, but can check. For the rest it is all on the same network and subnet, wired together through a couple dumb switches, nothing complicated.
Now as to secure connections and FQDNs needed. I hope that is not the case here, but the PS5 might require that. This would be a real pain though as it would mean that I have to create both a valid FQDN for my NAS as well as a related certified CA Certificate, ugh. A lot of hassle for something within a local home environment.
Maybe a question to get answered here might be, is the anyone that currently has PLEX client working on a PS5 at all, and secondary in combination with a Synology NAS hosting the plex server? And if the answer is yes, what was required to make this work?
