USB Antenna Not Working on Dell Optiplex 760 Desktop?

I have the 4.2.2 package downloaded on my 18.04 system. It will not 'make' without errors. I feel confident that you then tried the 5 version instead and it also didn't build properly for the same reasons. Let's clean them out:

sudo dkms remove rtl8812au/5.2.9 --all
sudo rm /usr/src/rtl8812au-5.2.9
sudo dkms remove rtl8812au/4.2.2 --all
sudo rm /usr/src/rtl8812au-4.2.2

There is an rtl8812au-dkms package available in the official Ubuntu repositories. It installs without error for me in 18.04. I suggest that you try:

sudo apt install rtl8812au-dkms

Reboot.

EDIT: We see from your modinfo that the installed version doesn’t actually cover your 0bda:a811 device. Let’s remove it and try a different driver file:

sudo apt purge rtl8812au-dkms
sudo apt install git
git clone https://github.com/abperiasamy/rtl8812AU_8821AU_linux.git
cd rtl8812AU_8821AU_linux
sudo make -f Makefile.dkms install
sudo modprobe rtl8812au

This driver clearly covers your device:

modinfo rtl8812au | grep A811

alias:          usb:v0BDApA811d*dc*dsc*dp*ic*isc*ip*in*
alias:          usb:v0BDApA811d*dc*dsc*dp*ic*isc*ip*in*
alias:          usb:v7392pA811d*dc*dsc*dp*ic*isc*ip*in*

You should be all set.