That's pretty simple and I use exactly the same setup at the office even with multiwan:
1. Delete everything you tried to get this connection going as it apperently doesn't work.
2. At system>advanced uncheck "disable nat reflection" at the bottom and save (this will make your public IP portforward available for the internal lan clients)
3. At firewall>nat hit the
- Icon and add a portforward for
external adress: interface Interface,
External Port Range: HTTP - <empty>,
NAT IP: <local IP of the server in DMZ>
local Port: HTTP
[X] Auto-add a firewall rule to permit traffic through this NAT rule
4. Save and apply
It should work now.