This could be a range of issues, all depending on your hardware. Its not really an issue related to Coldfusion. I would recommend asking on a forum that gives help for port forwarding / server management.
The other obvious thing I can add is that I assume you have checked the (windows) firewall on the server itself. Is it active and if so, does it also allow port 8080
In IIS, go to your website -> bindings and make sure you have 8080 listed as a port. IIS is the one doing the listening, not CF.
You may want to check the firewall on the server as you can set rules in here for local / domain traffic and external traffic.
Have a look at the MS guides Add or Edit Firewall Rule (for Remote access) or https://www.vultr.com/docs/how-to-open-a-port-in-windows-firewall-on-windows-server-2012 (for 'PUBLIC' access).