On your Master Controller, go to Security, Authentication, L3 Authentication, then Captive Portal Authentication, then choose the profile you are using for your Guest network, then you will see Login Page and Welcome Page on the right side. This is where you can point it to another page, provided that it exists on the CPPM (if that is where you want to point it to).
Hope that helps! Please give Kudos if it does work for you. Thanks.