FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 196076
When a FortiGate is behind an ISP that provides a dynamic IP address via DHCP or PPPoE, it is necessary to use an IPsec VPN dial-up client configuration on that device.

When using a route-based IPsec VPN configuration, Phase 2 or quick-mode selectors must be defined with internal/protected subnets to allow the IPsec VPN dial-up server configuration on the peer FortiGate device to dynamically install routes on its end.

When configuring a route-based IPsec VPN dial-up client configuration, it is necessary to define the internal/protected subnets as sources via a Phase 2 selector. It may be necessary to click the Convert to Custom Tunnel button to see the Phase 2 Selectors view.

For example, if the internal/protected subnet from which traffic is sourced is then the following Phase 2 selector needs to be defined:
Local Address:
Type: Subnet

Remote Address:
Type: Subnet
Value: (or specific subnet on the peer's protected network)