Download realtek r8168 linux driver from official site
https://www.realtek.com/component/zoo/category/network-interface-controllers-10-100-1000m-gigabit-ethernet-pci-express-software
Unpack source file
Replace orginal Makefile with this file
Put this source to 'package' folder of OpenWRT SDK
Build(make menuconfig, make defconfig, make)
0xc0000142/openwrt-r8168
Realtek Gigabit Ethernet RTL8111X / RTL8168X /RTL8411X PCI-e Driver for Openwrt
CMIT