is NAT required on FortiExtender 40D ??

Hello, is it compulsory to use NAT on the policy between the Fortigate and a FortiExtender 40D interface ?

The reason I ask is because I can only receive external traffic if I use NAT on outgoing traffic.

in other words, can a FortiExtender 40D be used for a 'private IP' WAN circuit to preserver private IP subnet addressing - or MUST it NAT out to the ISP's address range ?

Perhaps the higher models of FortiExtender provide this functionality ?