Question regards to Draytek vigor 2860ac vpn setup connected behind another router/gateway

[smallhickup]

I have a draytek 2860ac which has 4g/3g USB Dongle Mifi made by Huawei connected on USB2 (WAN4). Huawei mifi act as a gateway with ip address 192.168.8.1 and WAN4 ip address on Draytek is 192.168.8.100.

I have setup VPN remote dial in user using L2TP with IPSec. Under draytek admin portal, VPN and Remote Access -> Remote Dial-in User, I have subnet as LAN1. LAN1 on draytek router has ip address 192.168.1.1.

Now I tried port forwarding from huawei mifi to router as shown in the pic. Huawei mifi portal informs it as invalid as in the pic.

My vpn setup is not valid. I can't seem to get the VPN to work. Anyone has any suggestions to get it to work.

Thanks.

 

[smallhickup]

Should I port forward it to WAN4(192.168.8.100) on Draytek rather than LAN1(192.168.1.1) on Draytek?

Huawei Model I have is E8372 Wingle.