I have a Galaxy S5 and Asus RT-AC56R. Was having the same problem with my S5 not using the 5GHz band. I use MAC address filtering and also like to hide my SSID once the profile is saved on my phone. For some reason my S5 thinks the 5GHz band is turned off when my SSID is hidden. Shouldn't be a problem once the profile is saved on my phone. Worked like that on my old router and works on the 2.4GHz band. Anyway, my S5 is using 5GHz just fine with the SSID visible.