There are several Modbus libraries available on the internet.
Some of them work others do not work so good.
For my US customer I have modified it as such that it runs Modbus on a Controllino where the Modbus is the master and the device is the slave on RS485 RTU. The library has a limited support as the device is rather basic. The control of the bus runs from the interrupt handers. If interested, you can PM me and see if I can help you.
I did have the sensor connected to the PC through a RS485 To USB. I think it success. At least i got some data that seems valid to me. The problem is that when i tried to connect it to an arduino or Contollino Maxi. It failed. So, it would be great if someone could share some example that successfully connect a sensor to arduino or Controllino Maxi.