The built in web server runs on the port you define on the Configuration / Web Server Basic page
The server delivers files contained in the Data Directory (default: wwwroot)
Computer attached directly to the internet
here the computer running AbelCam is connected directly to the internet.
The computers IP number is accessible from the internet, AbelCams installer configures the windows firewall to allow access from the internet and you don't need to do anything for AbelCams web server to be accessible.
Computer on internal network
here the computer is connected to an internal network. Its IP number is private, not accessible from the internet.
You need to configure the router/gateway to forward port 8080 to the computer running AbelCam (Computer 1).
If you want to use windows media streaming, you need to forward the port(s) configured for the video server as well.
Depending on the model and configuration of your router, AbelCam on Computer 1 will only work as http://192.168.1.100:8080/, not as http://18.104.22.168:8080/
If for some reason the port used on the router is not the same as the port used for AbelCams web server, you need to configure the external port to match the router configuration.
Information on how to configure a router can be found on www.portforward.com