If you're using Windows 7, you should try the updated drivers from the Support pages: EU or US depending on your region.
If you're using Windows 10, try the driver for the ASUS PCE-AC68 or the Broadcom BCM-43xx Wireless Lan Drivers Version 7.35.338.0 WHQL.

This is an old thread now but I'm answering to help with searches.

Since you said you were on Windows 7, my followup question would be: how do you know you don't have an AC link?

Windows 7 isn't AC aware, as it is much older than the 802.11ac implementation, so none of the Windows components (wireless network control panels, netsh, ipconfig, etc.) know to report "ac" as a radio type. That's okay, it doesn't have to be AC-aware; the card still makes an ac link if available and if you connect to a 5GHZ AC-supporting SSID. It's a cosmetic issue, not a functional one. Typically, Windows 7 will report the radio type as N (the best it is aware of,) but will correctly report the actual link speed in Mbps. For example, if you have a 1300Mbps link.. you would have to be on AC as N won't go that fast.

Windows 8.0 also isn't AC aware. Windows 8.1 and up will correctly report AC in the radio type for netsh.