Hello,
Can you describe your use case? Do you want that router take control of slave device or control commands will be send from remote server using ModBus TCP, MQTT or something else?
If you will be using ModBus TCP master, then go to Services -> RS232/RS485 -> RS485 and configure ModBus gateway and allowed IP's.
If you want for router to take control of slave device, then RUT955 ModBus RTU master functionality can set slave device registers only via Alarm events.