Airport Express 802.11n not working with modem - RCA-DCM425.

My new Airport Express 802.11n not receiving valid IP address or DNS server address(es) from my modem, an RCA-DCM425. I have done a hard reset on the airport express, Airport utility recognizes it but when it tries to set it up it runs into two problems:

1. Your Apple wi-fi base station does not have a valid IP address.

2. No DNS Servers.

I am replacing an old Airport Express 802.11g.

Any advice?

cheers