Wifi setting

It's the network that decides. To my knowledge there is no setting to force only one type of WiFi.
 
I set up different ssid for 2.4 and 5ghz. I only connect to the 5ghz network.

But on the phone,I don't think that you cast choose
 
Yes, if you know which wifi is using 2.4 GHz and 5 ghz then pick either one you want to connect to.