Have you ensured latest firmware on your Asus RT-n66u?
Try switching out the network card on your Ubuntu server ... or if it is built-in to the motherboard, just add a new card. Preferably something that has a very long history of reliably working with Linux (like an Intel "server" class NIC). I bought this one a couple years ago http://www.newegg.com/Product/Produc...82E16833106015
and it has worked wonderfully for me under CentOS and Ubuntu. It is a dual port NIC ... but this single port one has a ton of good reviews that reference Linux: http://www.newegg.com/Product/Produc...82E16833106121