Hello @Dmytro Seliverstov ,
Unfortunately, to reduce resource usage and ensure long-lasting API/ABI compliance, the IoT Hub C SDK embedded within the Azure Sphere OS is compiled with the flag NO_LOGGING set.
You will be able to see from mqtt_client_set_trace in mqtt_client.c (which is called as part of OPTION_LOG_TRACE) that this functionality is disabled when NO_LOGGING is set.
Many thanks