There are quite a few topics, in the forum, about installing that driver …
Considering you still have an internet connection on your machine, either via LAN or via Phone Tethering, you first have to install the linux headers for your installed/running kernels. To do that run this command:
sudo pacman -S $(pacman -Qsq "^linux" | grep "^linux[0-9]*[-rt]*$" | awk '{print $1"-headers"}' ORS=' ')
then you build/install the driver with:
pamac build rtl88x2bu-dkms-git
Reboot the system and your adapter should work.