Can anybody please help me!!

My company has bought 2 of these routers and need help in trying to understand how the DMZ works.

We have 2 ADSL connections from different ISP's and have them in Wan1 and Wan2 both setup as static. Wan1's input is from a straight ADSL modem, so the 3300v has an outgoing IP address. Wan 2 is connected to a Draytek 2800 which contains the ISP user/password and is connected to the 3300v with IP address 192.168.1.2.

3300v lan is setup as 100.100.0.250 and DHCP server as 100.100.0.100~100.100.0.199.

All users in the office connect on the DHCP and we have a linux box that we need to serve webpages to the outside world. I have tried to connect the linux box in the DHCP but then found that the firewall rules of block lan to lan by IP does not work and it needs to be behind a DMZ.

Wan ports 3 and 4 are spare on the 3300v, and I connected the linux box to Wan4 to setup my DMZ.
Linux box has static IP address of 192.168.0.3.
I have tried both ways of connecting in NAT mode and Routing using 192.168.0.3 and using routing mode with Wan1 and 2. No success.
I have tried Nat DMZ host with private IP and IP alias and also address mapping. To be really honest I have not got a clue and the manual does not tell me anything. I have downloaded the up to date support manual but this didnt explain anything.

I think I am close and its something really simple I have missed, but I dont know what.

Hope someone can help

Simon