Make sure both ends of your ethernet cable are firmly attached. Also, try alternate cables/ports to see if it makes a difference.
Look in your BIOS. There should be option to disable the network adapter.
See page 3-15 of the manual:
https://dlcdnets.asus.com/pub/ASUS/...9_ROG_STRIX_B350-I_GAMING_UM_WEB_20171030.pdf