The following topic on Arch is marked solved
→ [SOLVED] Hardware issues with new MOBO - Gigabyte X870E / Kernel & Hardware / Arch Linux Forums
A similar topic on Manjaro forum - marked solved by using r8125-dkms from AUR - be sure to have the correct kernel headers synced
→ [root tip] [How To] Kernel - Headers - DKMS
→ Unable to use Ethernet (Realtek RTL8125 rev 0c)
sudo pacman-mirrors --api --set-branch unstable
sudo pacman -Syu base-devel linux612-headers
pamac build r8125-dkms