Disable dual transmission

Callie

Hi - I have a Post Office router, it transmits  WIFI on two channels and I want minimal WIFI so would rather disable one but can't find how to do it.  There are some settings that seem to allow me to select 1 channel but it doesn't make a difference.  Can anyone point me in the right direction please? Thank you! 

Greenlight

How do you know / What's the "two" channels the Router uses?
You can check "channel width" settings and set 20 MHz as the value.