Cisco 899 LTE to Fortigate FD200 - MPLS Primary and 4G backup

Hi guys,

Looking for some help. I want to use a Cisco 899 at remote branches with a MPLS connection as Primary and Cellular 4G with (GRE over IPSEC) as Backup to Fortigate FD200 at the Head Office

Was just going to use gateway of last resort for Cellular interface?

Is it possible to create IPSec gateways between the Cisco 899 and the Fortigate or should I be doing this another way?

Any help is appreciated.

Cheers