FortiWiFi 30E 3G4G and no route to SIM

I've deployed this FWF with a Vodafone SIM. My SIM connects to the internet and correctly obtains an IP address/gateway. From LAN I can ping the public IP address of SIM but I cannot route to internet... I cannot ping the gateway of the SIM.

If I put the SIM in a 50$ D-link 3G4G router it works like a charm...

I've tryed with Vodafone specific fw of the modem without results.

Do I miss some configurations in the firewall?

[Upgrade 2017/04/08]

I discovered that the obtained IP address of the SIM is on private network 10.x.x.x on Fortigate while it is a valid public address on the other routers... Why on fortigate does it take a private address?? 

Thanks,

Graziano.