[AC68U] OnePlus 3 cannot detect 5GHz SSID

My OnePlus 3 model A3003 phone running Android version 6.0.1 Marshmallow simply cannot see my 5G SSID. I know that 5G works fine as my laptop HP Elitebook 8440p with Intel Centrino Ultimate-N 6300 AGN running Windows 7 64bit SP1 works fine. The router is running an RT-AC68U running factory firmware 3.0.0.4.380_3831.

I've tried setting wl1_reg_mode=d as per...

[AC68U] OnePlus 3 cannot detect 5GHz SSID