No ethernet connection available in Ubuntu 20.04
i had the same exact issue. i was missing drivers for the Ethernet.
Go to Realtek site from the actual Ethernet provider. Download the "2.5G Ethernet LINUX driver r8125 for kernel up to 5.6" and follow the install instruction.
You should be able to use wired connection once your done.
This driver exists on the ubuntu repositories, the package is called: r8168-dkms. Yes, this works for the realtek 8125 wired ethernet connection.