Okay so I ran into this issue, after setting a manual channel of the 5GHz network to 44, my PCE-AC68 could find the network but not connect.
Antennas connected directly to the card rather than to the wired antenna mount. I rolled back my drivers to the ones shipped with the card, no use but the utility worked. Updated drivers to latest, utility doesn't work and Windows still wouldn't let me connect to the network.
So after some nerdraging and google I read somewhere that HDMI cables near wireless antennas can sometimes disrupt the 5GHz signal, never heard that before and was skeptical at best. Changed PCI-E ports to above my graphics card to keep the antennas free of cable interference, no luck.
But then I decided to use the antenna mount, said and done, got it out of the box and placed it on top of my desktop free of wires and anything else that might disrupt the signal.
Lo and behold, I suddenly have a perfect connection to the 5GHz band with latest drivers from ASUS, if you haven't tried the antenna mount already, do it and place it on your desktop where you see fit. It seems that the 5GHz band is way more sensitive than the old 2.4GHz band, especially to poorly isolated cables, in my case a cheap HDMI cable.