Rt-ac66u connection stuck in 20MHz mode

[bkoch709]

I have an Asus ROG laptop that I've recently upgraded the internal wireless card to an Intel 7260HMW AC radio. I just bought the RT-AC66U router yesterday, and brought everything up. However, I can't connect at AC speeds. It's stuck in 20MHz mode (max speed=144Mbps). Same thing happens with the Asus USB-AC53 AC1200 wireless USB stick. However, if I go to my back room with a Dlink AC router (in AP mode, not router mode), both adapters will connect in 80 MHz mode at 433 Mbps. What am I doing wrong?
The Asus router 5GHz band is configured for a different channel than the DLink router/AP, channel bandwidth is set for 20/40/80MHz, wireless mode is N/AC. Tried the 2 newest firmware versions with no change. About to do a factory default, and build again. What is up with this router?

 

[RMerlin]

The issue has been reported many times on these forums already - do not use N+AC mode, use "Auto" mode instead, otherwise your router will only allow 802.11n. This is a known bug in Asus's webui.