In order to display the webauth, the wlc will hijack the home page of the user and will try to resolve the page first. If the page can be resolved, then you will get the splash page. If not, then no. So first make sure DNS is working and also make sure the users home page is not an https site but they are using http. Plug a wired device onto the same vlan for the guest wireless in the DMZ and verify that it works for a wired device.