Setting up a router

I have a server based network and I’m trying to setup a router to share DSL throughout the network. It says to choose obtain IP automatically, which I did but on the server it doesn’t have that option, so I have to keep using the existing IP. So when I try to run the setup for the router my network stops communicating with the server, because they received an IP not compatible with that of the server. What can I do in this situation.
Thanks for your input.