Use different Port

Hello everyone,

this is my Scenario:

Internet -> Router(static IP) -> Internal SharePoint-Server Hosting a Web Application on Port 80.

Now Port 80 is already used ob other Servers. So I configured the router to forward Public Port 8080 to the Port 80 of the internal SharePoint Server. Well, the router does it, but it is not working because SharePoint always removes the Portnumber from the Url.

I enter: http://mydomain.com:8080 => then the URL changes to: http://mydomain.com/sites/test/pages/Home.aspx

So it wants to open my homepage in the test site collection, but it removes the port number. Is there a way to get this Working? Using google told me that

1. Alternate Access Mapping or
2. IIS Host Headers

can help here. But I don't know what is the right one for my scenario here and I really don't know what I need to setup to get this working. Is it even possible or do I need to use the same public Port like the Server Port number?

Thanks!
Regards
Christian