O:Here are some suggestions:
1.Make sure the "Onboard LAN" option was enabled in BIOS.
2.Make sure the LAN driver was installed properly. Please check it in "Device Manager".
3.Update the BIOS of your motherboard to the latest one.
4.Try the newer LAN driver from ASRock or chipset vendor's website.
ASRock: https://www.asrock.com/support/Download.asp
Realtek: http://www.realtek.com.tw/default.aspx
NVIDIA: http://www.nvidia.com/Download/index.aspx?lang=en-us
VIA
5.Change another LAN cable. (For Gigabit LAN, please make sure your LAN cable could fully work with Gigabit LAN.)
6.Reinstall the operating system.