This thread might be very helpful: Linksys WRT3200ACM only transmits with 20 MHz on the AC radio
It addresses:
- Why you see 36 when set to 80 MHz
- Information regarding DFS and regulatory settings
Incorrect, many countries require DFS, not just the US-FCC. Changing some radio configurations (e.g. DFS) can cause your device to transmit with a configuration that is illegal, causes interference, etc.
If you're allowed to use WiFi without DFS in your nation, that will be configured by changing the country setting.