Hi safabio,
I also tried to used mosquitto.org test broker but I haven’t managed to make it work yet (I even managed to make the chip crash!). Anyway, using part of the example provided in the AT reference document I’ve got a working sequence of commands :
AT+KMQTTCFG=0,"broker.hivemq.com",1883,4,"BX3105",60,0,0
+KMQTTCFG: 2
OK
AT+KMQTTCNX=2
OK
AT+KMQTTSUB=2,"sensor/#",0
OK
+KMQTTSUB: "sensor/humidity","25.00"
+KMQTTSUB: "sensor/temperature","24.00"
+KMQTTSUB: "sensor/temperature","27.00"
+KMQTTSUB: "sensor/humidity","28.00"
+KMQTTSUB: "sensor/temperature","25.00"
+KMQTTSUB: "sensor/humidity","31.00"
+KMQTTSUB: "sensor/humidity","18.00"
[...]