Fortiextender 40D WAN failover configuration

Hello, I have just retired a Cisco 1921 with HWIC 3G failover card with a Fortigate 60E and FortiExtender 40D for the same purpose.

However, when I tested WAN failover to the FortiExtender 3G mobile circuit, I received a public IP address on the FortiExtender interface from our ISP. Before with the Cisco configuration I would always receive a private IP address which enabled us to route to our WAN.

Is there any obvious reason why I am receiving a public IP instead of a private IP when the FortiExtender fails over to cellular backup ?

Thank you kindly.