Go to the Realtek website 387
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" and follow the installation instructions.
Go to /Downloads directory. Extract the Tarball.
In Terminal, go into r8125-xxxx folder and run the autorun.sh
$ sudo ./autorun.sh
Everything should run automatically and you should get ethernet connection now.