Hi Erwin,
Unfortunately MQTT(-SN) over NB-IOT as of yet is still unavailable for the iotcreators SIM cards this will be available shortly.
The manual kindly referred to by @Stefan-de-Lange is not related to IOT creators but to cloud of things which is a different offering entirely.
MQTT over LTE-M is available with these sim cards as can be read here. But this won’t help you with your NB-IOT modem.
Alternatively if you don’t like to wait I would suggest a different type of SIM offering (prepaid) we have also with a free plan here this has the MQTT-SN capability.