Hi,
My router wifi is 842L and wifi adapter card on PC is 548 (N300).
PC used Win 10.
I set 11n and Chanel with = Auto 20/40 Mhz but my PC just show speed < 150Mbps (20 Mhz). 
I want only setup Channel Width = 40 Mhz for 2,4 Ghz.
Who know setting it ?
Thanks,
What is your adapter on your PC? It might not be capable.
I had a 2016 laptop that was $300 and it only did 20 Mhz and was capped, but the wire was 1 Gb.
My $700 2008 laptop wireless could do 300 Mbps but only 100 Mbps wired. So go figure.
So we should start with your machine details...