Last week, I faced this weird problem. When my laptop is not connected to a power source (not on battery), I could not connect to LAN network with my LAN cable, but Wi-Fi worked fine. I struggled a lot for a week and then I found a solution in the internet.

All you have to do is to execute this one liner in your terminal.

echo on > /sys/class/net/eth1/device/power/control

Here eth1 corresponds to my second ethernet interface. It might vary from machine to machine. And if the directories eth, device and power dont exist, you might have to manually create them.