Mercusys 1200 router not connecting without WAN cable (check post)

When i plug in the WAN,the router ip site doesnt get recognised (because the LAN isnt plugged in). I dont have 2 ethernet cables ports,what am i supposed to do?

Here are my PC specs,if it has anything to do with this:

-GTX 860 TI

-Intel core I7 8800 kt-