I am running an ISP provided (must use) cable modem/router and an Asus RT-68U router with Merlin firmware (connected to the WAN port of the primary router). I would like to connect a USB disk drive to the USB.3 port of one or the other router (not one on each!) and access it from within the LAN and from the WAN side, at any time. A number of rather simple approaches like trying different NAT settings, DMZ etc. failed. Is it simply impossible, or am I just missing something ?