I dont have a clue

Thought set up would be as easy as the videos. Have deco 5 I can get thru setup until it says "no internet connection" I am plugged into an Att router/modem. I had it working at one time but went to move it now I cant get anything but "no internet" my computer works on the line. Try mutiple resets. When it says " cant connect" the light turns solid blue and stays like that. Getting so frustrated I have collected everything to ship back. PLEASE HELP. Do I have to do a passthrough on the att router ?? It worksd the first time without doing that.