Hi Friend,
I am sure there will be some restriction for the vlan which you might have configured for your guest wlan.
On the switch or on the router but somewhere. Controller just looks the wlan from the client and assign the vlan which is mapped to that wlan and nothing more than that so to have an access to guest wlan for the internal network or not you need to have some access filter for that vlan which is assigne dto guest wlan somewhere on your network.
Can you post the config of your switch and router so that we can have a look?
Regards,
Ankur