OK First click the Wireless Tab then Performance Enhancements.
Enable Band Steering. This will more than likely do the job and get you going fast. If it does not then you may have too old an adaptor and it won't keep up to your internet speeds from the modem. The advantage of 2.4 GHz over 5 GHz is distance. You can be farther away from the modem and still get into it. For example if I am on 5 GHz I can not get into the modem from the laundry room but on 2.4 GHz I can. In fact I can get into my modem outside the building on the bench out front. I have an older PC (4 years old) and it has a 2.4 GHz card in it only. There is No Blue Tooth or anything. Just the WiFi card. This setting allows me to go up 35 Mbs on the WiFi when without the setting on it only gets 1.2 Mbs speed. The Shaw Cable Modem I have is set up as a default to have it off. Why? I have no clue as it limits the speed dramatically. Several other brands of modem do the same thing but most fortunately do have it enabled as a default.