Ooohh! Couple of things you need..
1: I assume LAN, DMZ, and WAN are 3 different Security Zones.
2: Create 2 Security Policies ( Firewall Rules ) to Permit traffic from LAN to WAN, and DMZ to WAN.
3: Create a NAT Policy ( Source-NAT/ PAT) for LAN and DMZ Subnets, Translate them against the IP assigned on WAN Port
4: Create a default route (Static route ) pointing from your internal zones ( LAN, DMZ ) to WAN, point it to your ISP gateway ( I assume this info was provided by ISP )
Let me know if this won't work.