Okay, it appears to be a USR-WIFI232-C device.
I’m afraid you chose the wrong one really. Some of these USR gateways have built-in Modbus features, some don’t. This one doesn’t.
I use USR gateways myself (wired models) and chose the Modbus types - these work great, you just set up openHAB for Modbus-TCP and the gateway converts to Modbus-RTU for multiple slave devices.
USR-W610 would be the current WiFi model you want for that feature.
There are other very similar manufacturer’s products.
I can’t help much if you are stuck with what you already have.
As a Wndows user, I’d need to use a manufacturer supplied driver to create a virtual COM port on my system, and then setup openHAB to talk Modbus-RTU to the virtual COM port - the driver taking care of delivering that to the gateway in a way the gateway can use.
I don’t know how you do that on a *nix type system - I see you are trying to use socat but I do not know if this gateway will ever understand socat messages.
You have to be careful about permissions when attempting any serial port connections, real or virtual, with openHAB. Make sure you look in openhab.log for connection messages.
The Modbus binding will not try to send something until/unless you have configured a poller Thing.
EDIT -might help
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