Liunksys WRP400 no internet connection

Hey all,

Kinds similar to closed thread http://forums.techguy.org/networking...wont-work.html
I have a Linksys WRP400 router which was working fine.
All of a sudden when I plug in the cable from my ISP's modem to the 'Internet' port on the WRP400 the router does not get an ip and hence no internet connection. All devices can connect to the router via WiFI/wired port but no internet to share as the router doesn't have the internet connection.

modem connection direct to any PC works ok.
Also if I plug in modem cable to any of the 4 LAN ports on the router I can use wifi from other devices, however this is not ideal as the Bluface phone wont work etc.

Have done reset and upgraded latest firmware. No joy.
http://www.cisco.com/en/US/docs/voic...L-18475-01.pdf is the manual.
It's set to " Setup > Basic Setup > Internet Connection Type > Automatic Configuration - DHCP" as before when it did work.

Any ideas on what I can test/try next.

Thanks for any input..