Networking – Port Forwarding Does Not Work with a Dual-Network Device

I have a home NAS with a dual LAN cable connected to two different subnets.

    - lan1: on the subnet
- lan2: on the subnet

And on this NAS server, there is a web service exposing the port 5001 that I want to access from my office.

As a subnet gateway router, you have access to the Internet. I have configured DDNS and the port forwarding rules on it.

    - port forwarding rule: ->

But the tests showed that it did not work … Debugging started

There is a PC with the address on the same subnet

  1. Try to access from the PC and gateway router, works well

  2. Configure a demo web service on the PC: 5001, change the port forwarding rule: -> PC: 5001, access from my 4G mobile network, works well

I am confused after this because I thought that port forwarding is a kind of special reverse proxy. What should work if:

    - the resource provider is accessible from the proxy
- the proxy is accessible from its own network address

Which both confirmed ok in the debugging process before. So, what should I look into to find the problem then?