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.
IPsec VPN failed to established when Sonicwall pointing to dynamic IP [i.e FortiDDNS]. Debug output on FortiGate shows, after second message is received by initiator 'ignoring unencrypted INVALID-COOKIE' and retransmit.
To address this issue, on Sonicwall side change the Peer ID [IPV4 Address] to be the FortiGate's private IP which is facing the NAT Router.
The Fortinet Security Fabric brings together the concepts of convergence and consolidation to provide comprehensive cybersecurity protection for all users, devices, and applications and across all network edges.