Router WAN IP: 184.108.40.206
ZyWALL WAN IP: 192.168.10.33
Configuration on the router:
Add a NAT rule for the router.
Allow L2TP services.
L2TP server: 192.168.10.33
L2TP service: IKE, NATT, L2TP-UDP
Configuration on the ZyWALL/USG:
IPSec VPN Gateway
IPSec VPN Connection:
The local policy is the NAT public IP address.
Assign a pool for the L2TP clients.
Use a smartphone or a PC to establish a L2TP VPN connection the to ZyWALL/USG.
Configure the NAT's public IP address as the L2TP server address on the client.