I went through enormous pain to figure this out, thought to share with anyone who could one day fin this useful.
Debian 13's default kernel driver (r8169) fails to recognize certain RTL8125 chip revisions found in GMKtec mini PCs. Ethernet interface never appears in ip link show, even though the hardware is present and functional. Fix: compile and install Realtek's out-of-tree r8125 driver via DKMS.