DHCP Gateway

Server is running NT 4.0 sp6 and is configured to use DHCP. I am trying to gain access to a DSL modem on the network from a desktop. When the server assigns a DHCP address, the gateway assigned by the server appears to be blank, as viewed by winipcfg from a desktop, and I cannot gain access to the DSL modem. When I use a static IP address, however, the gateway shows up correctly when viewed using winipcfg, and I have no trouble getting out on the DSL modem. Desktops running Win95/98, and the TCP/IP properties on each desktop have been set up with the proper gateway and the DNS server#s. Why is this happening, and how can I get the server to assign the proper gateway to each desktop while using DHCP?