I have an IIS server (Windows Server 2012 R2) that has a Default Web Site with an application underneath it. I can browse and use that application fine while on the local network or any local VLAN. When I try to access the application from outside the firewall I get a 404 - File or directory not found error. Obviously I am able to reach the server and website through the firewall or I wouldn't be getting this error. I would get a blank response from the site not reachable. I've proven this by disabling the firewall rule that allows access. It seems as though there must be some kind of permissions or authentication issue preventing access to the application but I don't know what it would be.
I also tried setting up a subdirectory of this site with the default iisstart.htm and iis-85.png files that Microsoft provides. I cannot browse this subfolder either. Same 404 error.