Hi,
You must set the NAC option to "ISE NAC" (it will work with ClearPass despite having ISE in the name), otherwise redirect will not work.
Also, redirect will only happen when you access a site/ip that is not whitelisted. So, if you are already at your page the controller will not redirect you to a new page on the same address, if it was whitelisted.
You can see if the RADIUS integration is working by looking at the endpoint details at the WLC to see if the state os WEB_REQD and the redirect URL and ACL shows there.
For your case, you also have to make sure you enables "MAC Filtering" on the WLAN.
Regards.