This may have an obvious, easy answer, but I'm new to OMV and for me router configuration -- especially port forwarding -- is something of a black art. So please forgive me if the solution is obvious.
The instructions for configuring the Plex Media Server (PMS) plugin at Plex Media Server > Setup Info says:
Forward the following ports from your router to OMV. These ports are used for remote access to your Plex Media Server.
- TCP 32400 (for access to Plex Media Server) [required]
- TCP 32443 (for SSL access to Plex Media Server) [optional]
But my network already has a PMS running on another device (a Netgear ReadyNAS), and I cannot disable it until the OMV server is completely configured and up and running satisfactorily. (There's also a PMS on an iMac, but it's usually off because it's only used for testing.)
The "Manual Port Forwards for Multiple Servers" section of Plex's "Troubleshooting Remote Access" page suggests using external (WAN) ports 32401 and 32402 for two different servers and forwarding them both to internal (LAN) port 32400. But because this might break the existing setup, before doing this I want to be certain I'm doing things right. Here are some things to consider:
- Eventually I'll use the Plex Pass version of PMS.
- Eventually I'll decommission the ReadyNAS, at which time the OMV server should revert to external port 32400
- Currently there's no sign of life for the PMS running on the OMV machine. It's enabled, but the Plex Web Client can't connect to the server, and the other Plex clients on the network don't see the OMV server.
- This makes sense, since Port 32400 is not yet assigned to the OMV machine. But before doing so, I need confidence in a roadmap for getting to the final state.
Could someone please provide such a step-by-step roadmap? 
Thanks.