I am using an RT-N56U with factory firmware (3.0.0.4.376.3754) as a secondary router wired via the WAN port to a LAN port on an RT-N66U running TomatoUSB-Shibby 131 which is connected to my ISP modem. The 56U is servicing a PC and a BluRay player on my second floor while the 66U services a PC via ethernet, a Raspberry Pi and a printer via wifi. As it is, the devices connected to the 56U have a double NAT and it seems I cannot maintain the connection of that router to the primary...
Problems using RT-N56U as secondary router
Problems using RT-N56U as secondary router