Hi,
Is the web portal going to be from the ISE or the WLC? If you set up the ISE as both the web authentication portal and Sponsor Portal, then you need to change the Web Authentication Type on the WLC to External Server with and type the DNS resolvable address of the ISE Sponsor portal that you may have created.
If you are using the ISE as the authentication server only and WLC as the the web redirect portal, then only the ISE IP address specified as the Radius server is sufficient.
Finally, you need to check if your DNS server is actually up