Hello,
Just to clarify, you're setting up the VPN connection on a Fortigate router?
In passthrough mode, our router shares the same IP address between the LAN device (Fortigate in this case) and itself. In this mode, the router provides some limited functions.
If you'd like the router to only act as a gateway between 4G WAN and Fortigate, then bridge mode might help in resolving this issue, as it completely eliminates our router from the equation.
Best regards,
DaumantasG