UPnP Portmap Table - Empty / Blank - Netgear Nighthawk X6 AC3200 Router

I'm looking for help getting UPnP to work on my router. I'm seeking to remotely access a music server ("sonictransport" - wired device #1 on this screenshot) that is on my network. The music software that I am using to connect to this server remotely continually returns an error message. I took multiple actions on my router, but none resolved the issue. 

Router: Netgear Nighthawk X6 AC3200

Firmware: V1.0.4.84_10.1.84 (updated today)

Action #1: I updated the firmware of the router today. The firmware is now current.

Action #2: I turned off UPnP on my router. Thereafter, I re-activated UPnP (see attached UPnP screenshot confirming this feature is active). I waited over an hour and refreshed the table several times. 

My music software is still unable to remotely connect to my music server.

Here is the error message my music software receives: 

{
"connectivity": {"status":"NetworkError","status_code":504,"error":"error: Error: ETIMEDOUT, response code: undefined, body: undefined connected? undefined"},
"external_ip": {"actual_external_ip":"98.aaa.bbb.ccc","router_external_ip":"null"},
"natpmp_autoconfig": {"status":"NotFound"},
"upnp_autoconfig": {"status":"NotFound"}
}

Here is the list of attached devices my Netgear router displays and my UPnP portmap table after taking the above actions.

Attached Devices

https://app.box.com/s/korch9gkqv1c2un9p2i282mgpsvjr47x

My UPnP Portmap Table

https://app.box.com/s/6f9mzwp00nxik0e3u7amud2ca9yvgg4f

While this router is seven years old, it still meets all my needs except for this UPnP issue. 

I hope someone can help me resolve this issue so that I can keep this router out of a landfill and prevent me from having to buy a new router simply to get UPnP working properly. 

Thank you for your attention and assistance! -Mark