"Gateway" between two devices. How to redirect traffic to another device?

Question

Computer networks

In the existing system, there are 2 devices: a controller and a panel, which are connected to a local network.
Between them, data is transferred via the ModBus TCP protocol.
It is not possible to change the network settings of the controller and panel.
The panel asks strictly for the ip address of the controller.
The controller responds strictly to the ip address of the panel. It is

necessary to include a device in the network that will relay requests from the panel to the source controller and back.
There is a third-party controller with two Ethernet ports, but it is impossible to configure them for two different ip addresses on the same subnet.

Answer 1

Routing doesn't work that way. You need an L2 tunnel between two devices. If you are trying to connect these devices in one subnet via the Internet or as you have planned there.

Answer 2

You can try to reduce the mask by segmenting the network,
use the mask 255.255.255.128 on the gateway controller