I would check exactly what he got working - ie was he trying NTL cable.
If so, were back to the difference being the cable modem, but I've never heard of a modem working with a PC but not a router!
One other thing to try - browse to
http://192.168.100.1 (user - root, password - root) to bring up the modem diags and make a note (or screen image) of the info (obscuring the IP add). Then plug in to the router and see if you can still get to the modem config - it 'should' work even if it's not getting an external IP.