Native install of Linux mint 20.1 doesn’t have support for the 2.5G ethernet adapter on the B550 motherboard. Specifically the B500 AORUS PRO AX I am using. According to this post it will be natively supported with kernel 5.9 and above whereas the LTS mint kernel is around 5.4.
Fix is to manually install the Realtek 2.5G driver from the Realtek website.
Look for the download labelled
2.5G Ethernet LINUX driver r8125 for kernel up to 5.6
Unpack it and run the
autorun.sh to install. Don’t even need to reboot.
You may need to reinstall after a Mint update that touches network drivers.