If the wifi point is set to support all modes, then the client will connect. In addition, the speed can be set to 20Mhz or both 20Mhz/40Mhz. It will use 40Mhz if available or fall back to 20Mhz otherwise. There is no 40Mhz only option. So just because the network offers 40Mhz, doesn't mean it stops support for the 20Mhz also.
2.4ghz @ 20mhz width on channel 6. 5ghz @ 40mhz width on channel 36. Unifi Access Point: 2.4ghz @ 20mhz width on channel 11. 5ghz @ 40mhz width on channel 149. EDIT: Access Point doesnt support channel 149 so gone with 44. Doesnt conflict with 36 and is non DFS. According to the Unifi Controller settings, all channels except 36 to 44 are DFS.
On a non-MIMO setup (i.e. 802.11 a, b, or g) you should always try to use channel 1, 6, or 11. If you use 802.11n with 20MHz channels, stick to channels 1, 6, and 11 -- if you want to use 40MHz

Some brands use 150mbps / 300 / 450 to infer what bandwidth is being chosen. 150Mbps is the safe 1 20mhz bandwidth setting. The underlying issue is 40Mhz bandwidth is actually 2 adjoining full width wireless channels (ie 1 & 6) being used together to logically double throughput.

2.4 Ghz WiFi: 20 MHz vs 40 MHz vs 80 MHz. If you're using 2.4 GHz, the answer is simple. The best bandwidth for 2.4 Ghz is 20 MHz. In the majority of cases, using wide widths on 2.4 GHz isn't worthwhile. The performance tradeoffs from interference on overlapping channels will likely outweigh the throughput benefits. One possible exception to . 6 122 4 93 100 24 411 47

wifi 20mhz vs 40mhz