So I tried the VMG3925 with latest firmware, 3g swapped over but wouldn't connect and the 5ghz wifi would not work if set to auto on the 3925 GUI but if I set to channel 40 it worked so dont know if that is a hardware problem or firmware. So gone back to the Asus DSL-N55U and pulled the plug and the 3g dongle worked correctly. So it just seems when the line is reset by Admin on the ISP side that the Asus fails to re-connect.