You can try doing the following :
- Create an SVI/VLAN for the ISP and assigned an IP address in the 10.0.0.0/8 range ( is it possible for you to get a predictable IP address in this range)
(controller) (config) #vlan 10
(controller) (config) #interface vlan 10
(controller) (config) # ip address 10.x.x.x 255.0.0.0
- Create an SVI/VLAN for the wireless users and assigned an IP address in the 172.16.0.0/12
(controller) (config) #vlan 11
(controller) (config) #interface vlan 11
(controller) (config) # ip address 172.16.x.x 255.240.0.0
- Apply an static route with 0.0.0.0 0.0.0.0 through the address that you assigned the controller on 10.0.0.0/8.