Bridge Mode?

I probably should have asked what model of cable modem/router you have but essentially you would want the NAS drawing it's IP from the router and then if you want remote access to the NAS you would port forward to the IP assigned to the NAS by the router. Hope it helps.

I submitted a reply but not sure it arrived.  The router is a Linksys EA8500.  It isn't a combo modem/router.  The auto port-forwarding which is performed by the NAS (vs doing each port manually in the router) sees TWO routers...not just mine.  The research on this indicates Cox has a router on the line and so a BRIDGE needs to be established.  I attempted to get Cox to confirm whether or not the modem on their end is in BRIDGE mode for my connect but they didn't know what I was talking about.

In one of the many responses I found to your issue they mentioned that also - that Cox has a NAT further up the line. I have two routers on my system but one just piggy-backs off of the main one which is connected to the cable modem (the second one is just to add a VPN gateway). Your EA8500 is what should be issuing the IP via DHCP to the NAS and the NAS should just be set in Obtain IP Automatically mode. It should be noted that I'm not running an NAS here I've just had to configure them in the past (it's been awhile though, lol).

You might also try to install/setup the NAS with the WAN port disconnected from the cable modem and do your UPNP then reconnect everything as it should be and reboot equipment as needed. It can't hurt to try.