PC and xbox 360 to router

[mysteryman]

I have speedTouch 780 modem.router. My pc is connected to ethernet port 1 and my xbox 306 is connected to ethernet port 2. I am only able to use one device at a time as the router automatically sets both as the same IP address on the network. I cannot see any where to change the address in the router config menu. Tried resetting the router and upgrading firmware but makes no difference.

Any suggestions? Thanks