Asus PN50-E1 and 18.04 LTS - No network adapter

While several answers here appear to be duplicates, I don't believe any of them is exactly correct. I therefore propose a better answer.

With a working internet connection by tethering, USB ethernet or whatever means possible, open a terminal and do:

sudo apt update
sudo apt install dkms build-essential

Download the driver from the realtek website at: https://www.realtek.com/en/component/zoo/category/network-interface-controllers-10-100-1000m-gigabit-ethernet-pci-express-software Download the 2.5G Ethernet LINUX driver r8125 for kernel up to 5.6.

cd ~/Downloads/
tar -xvf r8125-9.006.04.tar.bz2 
cd r8125-9.006.04
sudo chmod +x autorun.sh
sudo ./autorun.sh

Reboot.