My airport express is not connecting to internet, problem seems to be reconciling the IP address of the router and the computer. Any ideas?

My airport express is not connecting to internet, problem seems to be reconciling the IP address of the router and the computer. Any ideas?

I called my cable company/internet provider and unplugged and plugged the modem and router cables and have no problem connecting MacBook directly to the modem via ethernet cable but no luck with Airport Express. Cable company explained that my wireless router needs its own IP address and should do this automatically but is not doing so. Said it is a common problem. Any advice would be greatly appreciated.