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.Solution
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 192.168.1.0/24 then the following Phase 2 selector needs to be defined:
Value: 0.0.0.0/0.0.0.0 (or specific subnet on the peer's protected network)