Help with Mac laptop, motorola modem and Apple router please

Hi all,
This one is weird!!! Trying to help a gentleman with his network. He has Comcast for internet, using their motorola modem and an apple router for the printer.
He had not used the network for several months and couldn't get connected. He gave up with Comcast, 7 different people, "paper clip resets"...(hard resets) and still no results. So he asked if I could help.
I started by unplugging both modem and router, and turned off the computer. I started the modem, got all green lights, plugged in the router, got a green light and turned on the computer.
All components worked last time he used it while in town.
Computer asked for the password (using airport), I entered it and shows it's connected, but no internet.
Here's the confusion....I'm clicking around, trying to see what the issue is, and BAM internet starts working, but now I have a flashing yellow light on the router no matter how many times I reboot.
I'm assuming the computer has somehow bypassed the router to give him internet, but obviously the printer won't work as this is going through the router which is attached to the modem.
Can anyone make heads or tails of what's going on here?
In advance, I thank you...
Ilene