Hello Guys,
When I assembled my PC with Realtek RTL8125B on Ubuntu 20.04,I couldn’t get my network to work.
My LAN was not working at all.To troubleshoot,I did lshw -class network and found the ethernet controller was “UNCLAIMED”
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.
Steps 
1.Download the “2.5G Ethernet LINUX driver r8125 for kernel up to 5.6” from https://www.realtek.com/en/component/zoo/category/network-interface-controllers-10-100-1000m-gigabit-ethernet-pci-express-software
2.sudo apt install r8168-dkms (this package was needed in order to run the r8125-90004.01 diver script with no errors)
3.cd /home/username/Downloads
4.cd r8125-90004.01 (Assuming you downloaded driver in this folder)
5.sudo chmod +x autorun.sh
6.sudo ./autorun.sh
7.sudo reboot