wrt1900ac internet connection problem

Hi,

I'm having a real basic problem; probably missing something easy, but I cannot get the wrt1900ac to connect to the internet.

This is my situation.

WORKING SITUATION: Cable modem connecting to PC over either yellow or orange or blue cable.  PC set to DHCP.  No problem.  wrt1900ac NOT involved.

DOESN'T WORK: ADD wrt1900ac to the situation: unplug the cable from the PC, turn off modem for 5 minutes, then plug modem/cable into wrt1900ac.  Turn on wrt1900ac.  Internet light goes blinking orange, then blinking white, then blinking orange, then blinking white, then solid orange.  FAIL.

I don't understand how this all could be true unless there is something wrong with the wrt1900ac. 

What am I missing?  Why won't the wrt1900ac connect to the internet?