L2TP Server Configuration

Dear Friends,

I want to configure the FG 200D as a L2TP server and want to connect 15 no. of vpn supported router L2TP VPN.

My Requirement is -

1. At fortigate 200D (5.2) i have public IP

2. At Remote Site Router (15 No.) no public IP - Router Model - Techroute TR1803 3G

3. Remote site routers are 3G SIM Supported Router.

Now Please share the step by step configuration for this setup...

Thanks..

JM