One of the most common issues in DNN are aliases.
Once you install DNN to a give domain, it will use it once you connect to the portal.
You can have several aliases in IIS, but they only redirect to that one domain.
So the fix is kind of easy:
- Download and run "SQL Server Management Studio Express" if you don't have it yet
- Attach the DNN database in your_directory/dotnetnuke_community_your_version/app_data/database.mdf
- Open table dbo.PortalAlias
- Edit HTTPAlias field