Connect your PC direct to the modem via an Ethernet cable and check your speed and connectivity. If all is good, use the same cable and connect modem to the router. Always use a good quality cable and be sure to make a good connection. I seen one poster say the cable that came with the router was bad so do not assume it is good because it is new. Always power cycle (modem, router and PC) between changes although not always necessary but does help sometimes.

@Shaptak 

 

Also, make sure the Ethernet cable from the modem is going to the blue port, may seem trivial but best to check.

I have the same problem for Archer C9 modem. "WAN port is unplugged !" I cable is working fine. But modem is not able to detect it.

@aravinth 

 

If you verifed the cable and modem work by connecting your computer, then the Archer C9 may have a faulty WAN port.