How to set up traffic routing in Zyxel keenetic?

Question

Computer networks

Hello! Help set up port forwarding.
The task is to forward the port to the web interface of the 4g modem.
There is a zyxel keenetik 4g II router (192.168.2.1). It is connected to the internet via a 4g modem that runs in ndis mode and has ip 192.168.8.1.
Through this Internet connection, the router connects to vpn (pptp configured on its server) and receives ip 192.168.5.18.
Tell me how to set up port forwarding so that when accessing a router from a vpn network to a specific port, it forwards it to the modem's web interface. As I understand it, this must be done using the route. Please tell me how to set it up correctly. Independent attempts did not give results.

Answer 1

This scheme did not work for me and here's why:
Let's say we figured out the routes and we can normally route the network behind the router and the necessary networks behind the vpn.
Next, we go to the firewall and set up port forwarding: for example, we want to access 192.168.1.2 and get into the modem's web interface. Ok, set up port forwarding, incoming vpn interface, ip 192.168.1.2, all ports, all protocols, to 10.0.0.1 (modem address).
But the scheme does not work due to the fact that the NAT router only uses its own local network.
The redirect works, but the modem does not have a route to the desired network, where the packet came from, and replies through the default gateway.
In short, you need to NAT connections from the VPN to the network that the modem distributes. How to enable this on Zyxel keenetic - xs, this question is also interesting.