Can't get RTL8125B working on 20.04

I had the same exact issue. Looks like you're missing drivers for the Ethernet.

Go to this link (which is from the actual Ethernet provider): 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.

You should be able to use wired connection once you're done.

The rtl8125b support has been added to the linux-next tree 6 days ago.

https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/drivers/net/ethernet/realtek?h=next-20200720&id=0439297be95111cf9ef5ece2091af16d140ce2ef

It will take some time to get into stable and then into Ubuntu.

You can try to install drivers from the Realtek site before Linux supports it natively. You'll need to disable Secure Boot in BIOS and blacklist r8169 to get them working.

The issue is fixed in the 5.9 kernel. Unfortunately Ubuntu 20.10 and 20.04.2 won't have this kernel. So a mainline kernel, or a backport is needed.