How can I force 40mhz mode on N900?

Ok, I have an issue with my WD N900. The wifi speed is limited to 144mbps. It is set on wifi n only mode, WPA2 Personal with 2.4 & 5ghz (20/40Mhz Auto). It seems like my router is only running in 20 mhz mode. Is there a way to force the 40 Mhz mode instead of the auto mode ? 

Btw I have used different wifi cards and they are all limited to 144mbps.

Thank you.

Have you checked your adapter settings? Some allow for 40 width, 20 or Auto, and also Fat Channel (channel bonding) tolerance. If I make the appropriate changes to my wife’s laptop adapter, it will connect at 300 (2.4, it cant see the 5Ghz band) on my N750, but won’t sustain the speed for long since our neighborhood has a number of other networks broadcasting. Since my speeds are 30/4, 54-144 reliably is sufficient for our needs at the moment.