deco m5 main cannot use lan cable to desktop or another deco... main deco to módem with ppoe, other port is using a lan cable to desktop. The deco cant resolver the IP. Anyhelp?

i use deco m5 first setup main to modem, other port using lan cable to desktop. salve deco use wifi to main deco. after one day, my desktop show unidentified network. i try my computer and laptop are