please make sure, that the new domain address is properly routed in DNS and IIS to your DotNetNuke installation site, before you create the new portal.

if you are adding a domain, the domain address needs to be located in a name server (either at the registering company or inside your LAN) and the IIS web server needs to forward it to the proper web site, if there are multiple installed, usually by assigning a host header value to the IP in IIS web site configuration. place a simple hello.html in the root of your DNN folder and make sure, it gets displayed correctly when putting yourdomain/hello.htm into the address bar of your browser.