if you are using TP-Link VR1600v just log in to the modem's interface http://192.168.1.1 and go to Wireless on the left panel under basic and just untick the 5g and click save it
Here's the screen shot
I want to do the same, but my screen does not show these options to me, the only option is:
Wireless network. I find out that I can go to the individual 2.4G or 5G to do the selection. Thanks to all. Much appreciated!