Sorry for hijacking this thread but I think this question isn't worth a new thread. Anyway, I updated the firmware of my parents' RT-N56U to 3.0.0.4.380_7378 and did a reset. Now the "USB mode" setting under USB Application -> 3G/4G stays on and cannot be turned off. If I turn it off and choose "Apply", the router reboots and after that, the setting is "on" again.
I guess this isn't very serious but does anyone know a solution, or has anyone else noticed this problem? Also, there isn't a "Dual WAN" option in the WAN panel anymore (though I'm not sure if there ever was in this model...).