I know you would probbaly found a way but I am leaving this comment for anyone in the future.
use the "MQTT gateway" under "Modbus" section, also set the Modbus slave to "on”, set the Modbus master on and add the Digital output status register 202 so that you can read its value, the MQTT allow you to send command to the device (you activated the device as modbus slave in Modbus slave option before), also activate the data to server option and set it to collect data from modbus. Now try to send this command " 0 65432 0 127.0.0.1 502 5 1 6 202 1" through the request topic you set in "MQTT gateway" you will receive OK message on the response topic you set in "MQTT gateway" you created. You can also receive the actual status of the pin (register 202) that you read through the "MODBUS master" section you created. And the "data to server" section will take the data from the "MODBUS master" and send it to the MQTT
here is some links to help do what I just said:
https://wiki.teltonika-networks.com/view/RUT955_Modbus
https://wiki.teltonika-networks.com/view/RUT955_Input/Output
https://wiki.teltonika-networks.com/view/Modbus_TCP_Master_MQTT_Gateway
I know it is a mess but it works really well.