Reference

https://community.fortinet.com/fortigate-3/technical-tip-ipsec-tunnels-not-connecting-after-upgrade-to-v7-6-5-or-above-220185

Situation

# IPsec預設DH group變更
The default IPsec DH group has changed starting with FortiOS 7.6.5.

Solution

[1]
Log in Fortigate GUI

[2]
# 檢查設定是不是預設值，Phase1要看、Phase2如果有開啟PFS也要注意DH Group
VPN > IPsec > edit tunnel

Check

diagnose debug application ike -1
diagnose debug console timestamp enable
diagnose debug enable