Configuring Plex Remote Access with NAS device

> I am running a Plex server on a NAS device [...]

   Did it come with any instructions for external access, say, regarding
port forwarding, or which ports it uses, or anything relevant, or were
you hoping that intuition would do the job?

> [...] when I press retry on plex remote access, [...]

   You're doing what, exactly, on what, exactly, where, exactly?

> When UPnP is turned on, the portmap table displays this:

   How much of that do you think is related to your Plex server?  (I
avoid UPnP, so I know nothing, but I doubt that 443 is, and the "Ext.
Port" values look (to me) like noise.)

> [...] 192.168.c.d

   There's no reason to hide any private addresses.

> 70.171.[x.y] (public)

   _That_ was what you wanted to disguise.

> External Port Range: (wasn't sure what to put here, so I tried both
> 49592 and 26176)

   Most likely, you want the same as the Internal Port Range, at least
until you learn what you'd need to do on the client to deal with some
other port.  Not all of the UPnP stuff applies to explicit port
forwarding.

   I know approximately nothing about Plex server access, but my quick
Web search (plex ports router) led to:

      https://support.plex.tv/articles/200931138

   For the usual problems with port forwarding, see:

      https://community.netgear.com/t5/x/x/m-p/1859106

But I'd guess that your main problem is a defective port-forwarding
rule.

> I have a similar setup. [...]

   No, you don't.  Compare "NAS device [...] connected to my router via
ethernet cable" with "hard drive connected via usb".

   You should start your own thread for your own, different situation.
You might want to include some information about what _you_ did to make
_your_ Plex server available to the outside world.  And the firmware
version of your R9000.  And to what it's connected.