Python3 is available on Teltonika Networks devices that have enough memory for it. RUT200 does not have much memory, so Python is likely not an option for this device.
However, you should be able to achieve your goals via shell scripting.
You can create a shell script where you can use UBUS modbus calls to send Modbus requests to your Modbus servers. Then, you can proccess the response the way you need, and publish it to your MQTT broker via Mosquitto.
There’s quite a few posts on this forum that contain useful information for your case. For example, the following posts might be helpful:
Thus, I suggest using the search functionality to find relevant posts.