That was before it switched to 5.6.4.2 firmware ?
Maybe because you need the older firmware branch?
There are two AUR packages for this dongle
from 5.1.5 branch
http://aur.archlinux.org/packages/rtl8821au-dkms-git
from 5.6.4 branch
http://aur.archlinux.org/packages/rtl8812au-dkms-git
But, from what i read, the one you need is rtl8814au not rtl8821au nor rtl8812au
That means this one:
https://aur.archlinux.org/packages/rtl88xxau-aircrack-dkms-git