20MHz for 2.4GHz is narrow band you can only reach max about 50-75Mbps on speedtest if you choose this setting.
Increase it to 40MHz you'll probably get max 125Mbps if you have a supported wifi client/device that supports it. Most newer equipment nowadays already support this unless you have very old outdated stuff at home that'll need Wi-Fi connectivity.
Then there's 2x2 MIMO on top of the 40MHz 2.4GHz Wifi which claims to oduble the speed with 2 parallel antennas.
Do remember that even though your router/AP supports newer technologies such as 2.4GHz wide band Wifi and MIMO 2x2, you need a client that matches it as well.
For 300Mbps plans above, it's recommended that you use 5GHz WiFi instead to gain the extra bandwidth and take advantage of the speed.
For 100Mbps plans below, 40MHz 2.4GHz will be good enough for most people.
My stock router at home is capable of AC1200, it makes sense to get a AC1200 rated wifi adapter or a matching capable device to take advantage of the speed which is 1.2Gbps via wireless.
Do note that speeds are theoretical speeds, give or take 20% overhead with published speeds on the tech sheet, that's what'll get in real time conditions.
You can put any widest band if you have no interference from any other WAP like from neighbor or your other WAP. I'd leave the 2.4GHz band for guess and old devices.
For the 5GHz band , it is different story because you have more channels. Unless your device can do link aggregation, you can only use either band only. So, no 1200Mbps total