Thank you very much for your request. The MQTT-SN CONNECT packet looks right so far and since you get an answer from the gateway, the communication works well. It seems that the registration in the cloud of things is not quite correct, especially the field ICCID. Quote from the user manual:
ICCID: The ICCID of the device (equal to the ID of the device)
Important here is the addition in brackets “equal to ID of the device”. In this case, the ID of the device is the IMSI and must also be specified when registering the device. Please try the registration of the device again with the IMSI in the field ICCID, then it should work.