Your router config looks like it should work and I successfuly port forward with ERX v 1.9 so I don't think it's the router. I use PLEX and default it tries to open up a port with UPNP which is usually enabled by default on consumer routers but is not enabled by default on the edgerouter. I'm not certain it will try the default port 32400 unless it is manually specified in the server settings for plex so I would try that to see if it works, if it isn't already set. Otherwise you can enable upnp if you want it
It would be under
set service upnp/upnp2 listen-on switch0
I don't currently use upnp so I'm not sure the best one to use or all the correct options.
