Cannot Sign Into Report Manager Over the Web (meaning using my Web IP Address for external access)

Running SQL Server Express/SSRS 2014 on my Windows 7 Professional computer.  I'm using Windows Security.

I can access Report Manager fine on my machine when I use either my machine name, local host, or internal IP address.

I'm Port Forwarding 80 to my internal IP address and also have my external IP Address as a Trusted Site in IE. 

I have also added http://xx.xxx.xxx.xxx/Reports_SQLEXPRESS to Report Manager URL and Web Service URL in Configuration Manager.

When I try to use the external IP address, I get the Windows Security sign-on but it will not take my user name and password. I put it in 3 times and then just get a blank page.

Is there an additional security step I need to take for external web access?

Thanks,

Joe