Remember the Cable modem will only assign an IP address to one mac address. You need to unplug the cable modem every time you swap it from a router to your pc. It sounds like you haven't reset the cable modem. I may be teaching you to suck eggs but if you don't reset the modem every single time it will refuse to communicate with anything other than the network card with the specific mac address it wants.
excellent point
however, it doesnt explain why the PC is not obtaining an IP from his router as well as the new one in the box. The PC is clearly not obtaining an IP automatically for some reason as the PC wont get an IP from a known working router.
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