@dmichaelj 

So, if you disconnect the QNAP server's Ethernet cable from the LAN port of the AX20 and instead plug it into one of the LAN ports of the AX73, then PLEX works fine?

Hi  @woozle

Thanks for the reply. I have connected it direct to my AX73 and I get exactly the same message from PLEX. It works for a few minutes (with a lovely green tick!) and then stops and displays the following message:

"Not available outside your network

Your server is signed in to Plex, but is not reachable from outside your network. Tip: It looks like your server may be connected to the internet through multiple routers or other network devices. Try connecting it directly to your primary device, or visit our support site for more information about troubleshooting this "Double-NAT" scenario."

So I'm guessing this is an issue wider than a conflict caused between two devices...

I have opened up a port on the router and when I click retry - again it works for a few minutes with a green tick and then displays the above message again. 
 

During which time, Plex server content can be stream on a device not on my network fine for a few minutes. But once the above message displays it can only be played outside my network through indirect play. So it seems the conflict kicks in after a few minutes....

I'm stumped! Do you have any idea what might be going on?

Thanks again,

D