GNU/Linux ASUS USB-AC56 Setup – Step-by-step Guide
How to Install ASUS USB-AC56 AC1300 Wireless 802.11ac Driver on GNU/Linux Computers – Step by step Tutorial.
And the GNU+Linux ASUS USB-AC56 Setup is based on the Realtek rtl8814AU Chipset Driver Installation.

GNU/Linux ASUS USB-AC56 Wireless Driver Installation Guide
On the Driver Download page, take into account that the Morrownr release is currently the recommended choice for this Dongle.