Slow Wireless Internet

I am having problems trying to get good WIFI speed using my Nighthawk x4s D7800  modem.  I am getting good wired speed (around 75 MBps) but around 20 mbps on wireless.  I am next to the modem and if I use a BT home Hub 5 I get 75mbps on wireless without problems.  I am using the latest firmware V1.0.0.48 for the D7800 , 

QoS and WMM are disabled and I already tried Resetting the modem. Can anybody help me? 

Thanks in advance.

Mario