Currently Unsure about something in my router settings.

(wasnt sure which tag) basically my download speeds were abysmal, like 1mb per second or sometimes 3.5mb, still awful then i looked at the router settings and found 802.11/g/b/n or sm like that, then i found 11m bandwidth which was set to 20/40 now i know about the older device compaitability and im certain all my devices support 802.11n or higher, so how should i do this or is there a better option to increase my speed?