Question about ASUS 4G-AC86U

Yes, it is possible to switch off the 5 GHz WiFi in the software. You can do this via the WebGUI or the ASUS Router App. In the WebGUI, go to "Wireless" > "Professional" and set "Enable Radio" to "No" for the 5 GHz band. Alternatively, you can make the appropriate settings in the app under "Settings" > "WiFi" > "Wireless Settings" > "Advanced Settings".