MQTT-SN is a protocol for data transmission (specification available here:
It works in the publish/subscribe manner. A device which wants to send data to CoT, needs to have implemented this protocol. The easiest way to implement is to use the Open Source Paho Client from von Eclipse.
CoT can only be addressed over the REST interface and therefore the connector is needed to convert incoming MQTT-SN messages to CoT messages.
There is a second important function of the connector. It interprets data, so for example temperature can be identified as such. Prerequisite for that is that MQTT-SN messages are being formatted according a predefined convention. How this is to be accomplished is described in the NBIOT-Protocol-Developerguide.
If MQTT-SN messages are correctly formatted and compliant to the convention described in the developerguide, then data is transferred into the Cloud of Things.
The mentioned IP address belongs to a private network, since it is the tunnel endpoint of the IPSec-tunnel from private APN of the CoT. This server is only acceible via the private APN.
For sending UDP packets to a server residing in the Internet, you need to have SIM cards with public APN (internet.nbiot.telekom.de). Since you have ordered only SIM cards for CoT, you are only able to send data to the CoT with IP 172.25.102.151:1883