Hello there! what a mess 
First at all, you are not even close to clear on how you are using the VPN. If you are running a vpn to the ENTIRE computer where you are running the plex, you are going to have a lot of issues. First, plex server will detect that your external IP is the one on the VPN and not your real IP.
Things to understand: You cannot map incoming port thru your vpn service provider. You can do that only if you manage your OWN vpn server.
If you are redirecting all your traffic to vpn you should be able to map ports to your plex but will definitely be a pain. You need to check if the ip address showed “public” at server settings/remote access is your REAL internet ip address (from your ISP) and not the VPN ip address But definitely will be your vpn. Here there is a post from someone that had this issue: Remote Access over different IP address - #3 by marove
Is my opinion that you separate the plex server from the VPN. Add a second network card, or use different machine, virtual machine, etc.
