I have 2 gateways in my network, then I have to route to the right gateway to access the plex authentication servers, to be sure that the direct access is correctly configured.
you know the plex IP subnet please?
Are you trying to run Plex behind a VPN? If so that is troublesome but possible, do a google search for it, there are many articles
I connect through a LTE network, then private network.
To be able to control a port translation from a public IP I do a reverse ssh tunnel.
Then I can access my plex from a public address.
But to trafic from my server to plex server doesn’t use the route from the public address. then I have to know the plex IP destinations to route it through the same public address.
When I don’t use the same public address, plex saw my direct like goes through my LTE access, but I can’t modify the NAT/PAT, it is a provider backbone router.