You need a VPN feature called split tunneling. A function of split tunneling is to allow traffic to be selectively routed via either the VPN or to bypass it. Unfortunately, as far as I know, NordVPN does not support this on any of their clients, with the exception of Android.
With the VPN enabled, are you still able to manually browse to your server using its WAN IP address and remote access port (from outside your network)? If so, you can try using a custom server access URL to advertise your WAN IP address to Plex for remote access to attempt to bypass the VPN for inbound connections. This support article has more information:
The URL you configure needs to be constructed from a few bits of information:
- The URL scheme (https://)
- Your WAN IP address, delimited with hyphens instead of periods
- Your the certificate UUID assigned to your server. On a Mac, you can find this by running:
defaults read com.plexapp.plexmediaserver CertificateUUID - The literal string “plex.direct”
- Optionally, the port you’ve specified for remote access, if not using the default (32400)
A full URL might look like this:
https://1-1-1-1.abcdef12345678890abcdef.plex.direct:32400
You may need to disable and re-enabled remote access, or restart the server after making this change. Also be aware that you’ll have to manually change this if your public IP changes. For that reason, it may be better to use your own custom domain name and certificate and dynamic DNS (or a static IP).