Sometimes you need to have the uplink enabled on the router too. Maybe that's it? The_Dude posts a good point. I have heard of service providers recording a router's MAC address and allowing only that router to function on the network. I'm on Comcast and we didn't have that problem when I recently switched out a router -- Cox Cable is safe too.
Have you tried a switch or a hub in place of the router? Tried to spoof the MAC with your router?
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