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.