TCP/IP Internet Connection

I am on a NT 4.0 network with 4 servers with an isdn connection to the internet. From my workstation, I can change my IP address and subnet mask to connect through my router to the isdn inernet connection. When I do this I cannot connect to my network. If I change my TCP/IP setting to assign an IP address automatically, I can then connect to my network but not the internet. How can I connect to my network and also maintain my connection to the internet.