Your gateway/router will have 2 ip addresses. On the wan side, it will be the ip given to you by your internet provider. If it is a dynamic ip that is given to you from a dhcp server, you should be able to configure the gateway to get the ip from the dhcp server. It will also have a static ip on the lan side. This ip is usually at the upper or lower end of available addresses. This address is usually 192.168.0.1 and the other machines will have a number within that subnet 192.168.0.xxx. You should have the option of giving each of your machines a static address or let the gateway act as a dhcp server for your lan. Your servers must have a static ip so that traffic can be forwarded to the correct machine. Let's say your server is 192.168.0.20. Then you will forward all http traffic to that ip on port 80. Apache listens for ALL addresses on port 80 by default so it will answer. You will need to setup ALL machines to use the lan ip of the gateway as both default gateway and dns servers. Your gateway should only go into configuration mode when you set your browser to its lan ip from within your local network. You can set it up to allow remote access but this is not set by default. I do not use the same gateway you have but this is a typical setup for most networks.
