The Raspberry Pi needs a static IP address address on your local network. DHCP assigns IP addresses for your computers in the range 192.168.1.0 to 192.168.1.255 more or less at random. All routers are a little different, but you have to assign a static address to the Raspberry Pi. In the DHCP setup there is an option to assign lan addresses to ports by server ports or MAC addresses.
This is where I would have set this if I was sure I would always have my Raspberry Pi on an ethernet cable.
I have assigned port 80 on the WAN port to be served by the computer at 192.168.1.98 on the lan. That means any requests to the router on the web port 80 will be served by the raspberry pi.
That would go in these boxes, but I thought I might want to make this a wireless device so I used the Mac address to assign the address.
Then we port forward the traffic from the WWW to our Raspberry Pi.
This is the result of running ifconfig. The address of the Raspberry Pi is 192.168.1.98 on my lan.
Web traffic is now routed to my Raspberry Pi.