VPN USING PORT FOWARD TO THE FGT INTERNAL PRIVATE IP

Hi,

I'm new to the VPN Config, to make it successful the WAN interface must get the Public IP from the ISP meaning to say that the modem need to be set to bridged mode so that the FGT will dial to the internet. My issue now, how if i want to config the VPN without changing the modem to bridged and port foward the Public IP to the Internal FGT IP so that i can create the VPN config. 

Can it be done?

Really appreciate the help here