multihomed server, NAT, Firewall, Win98

I have a client with a Multihomed PDC, IP1= 192.168.1.1, 255.255.255.0, 192.168.1.20 Gateway. IP2=192.168.2.1, 255.255.255.0, No Gateway. IPX/SPX,Netbieu, TCP/IP, we need these protocols because of specialised software running from the SERVER. Everything has been working OK with our static routes, until we are trying to add a NAT/Firewall(DI-704),IP=192.168.1.20. DSL modem with static IP. Clients on 192.168.1.xxx work OK for Internet. Clients 192.168.2.xxx can ping Gateway but cannot ping Internet addresses. Browsing is none functional. Please help.