Fortigate 30E 3g4g WWAN settings

Hello!

 Before I wrote this post I tried to find any information about WWAN settings on Fortigate, some tips how to configure it, but my search ends up here :).

 Long story short: I have to configure Fortigate 30E 3G4G. I'd like to have a backup WAN connection which uses SIM-card. When SIM card is plugged it, WWAN interface is down, it's set as DHCP. Unfortunately nothing happens, it works with my mobile phone, PIN is off. My internet provider for SIM is T-Mobile.

 Do you have any ideas how it should be configured? I couldn't find any guides.

Thank you in advance!