Well yes that is what I understand as well, but the only documentation I have is an example with som definitions:
I'm missing the prespective from the Azure IoT Hub perspective.
* <cookie> is that mandatory, and is that defined, other than "a 64-bit unsigned integer in range [0..264]). A cookie is ...". Can Azure application use any cookie value?
* <ip>, in the example, 192.168.1.1 is used. Not really useful for the Azure application right?
So my follow up question is How to configure the MQTT gateway (I assume you mean the RUT955 MQTT GW) to write a new value to modbus register from Azure?