Situation

Multiple IPsec tunnels established over one WAN IP.

Solution

[1]
Log in Fortigate WebUI > VPN > IPsec Tunnels > Edit "Dialup Tunnel"

[2]
# 指定Peer ID來對不同的Tunnel做連線
Authentication > Peer Options > Accept Types: Specific peer ID > Peer ID: "String_1"

[3]
# FortiClient需要輸入"String_1"來連線
FortiClient GUI > Remote Access > 三 > 更改選定連結 > 高級配置 > Phase1 > 本地ID > "String_1"