Fortigate self-generated traffic to Internet from external private address

Hello,

I have a Fortigate 3000 running V5.6.3. Its external ethernet interface, connected to the Internet router, has a private IP address.

The problem is that it cannot reach the Fortinet site because its subscription information (sandboxing, signatures, virus, etc)  self-generated outgoing IP packets are sent to Internet with its external interface private source IP address instead of the global IP it should use to be properly routed. 

I would really appreciate any example of help in how to perform SNAT for its self generated traffic to Internet.

I created an outgoing policy performing SNAT from an IP Pool containing the IP public address. And this works perfectly for some internal hosts needing some direct access to Internet. The problem is that I cannot find the way to do the same for the Fortigate self-generated traffic.

Regards in advance.

Albert.