http://networkengineering.stackexcha...efend-firewall
My specific problem is addressed on Section 7.4.1. I need to allow my internal server to use my router's IP when replying to a remote device, we need to define a NAT rule for this. Then, we also need to Allow traffic to go from my lan or DMZ into the internet, this means also a new IP rule.

In my case, configuration should add three IP rules like this:

SAT RULE:
Source: any as Interface, all-nets as Network
Destination: core as Interface, wan1_ip as Network (public IP, assigned to this router)
Service is http-in in my case
Webserver local IP goes on the SAT tab.
NAT RULE
Source: lan as Interface, lan-net as Network (or DMZ if you're using one, which you should)
Destination: any as Interface, all-nets as Network.
Service can either be "all_services", or http_in in my case according to the manual. I chose the later.
ALLOW RULE
Source: any as Interface, all-nets as Network.
Destination: core as Interface, wan1_ip as Network.
Service is again, http-in for my case
Hope this helps somebody.