MQTT Flex allows you to use the data services from any SIM card to send MQTT-SN messages over the public internet to the Thingstream broker. The MQTT Flex Thing that you created in the Thingstream portal provides the credentials that the MQTT-SN client on your modem requires in order to authenticate with the Thingstream broker.
MQTT Anywhere, uses the Thingstream global roaming SIM which provides a private secure APN. There is no need to set up DTLS or configure client authentication with MQTT Anywhere. Security and authentication are all handled by the SIM card
For more detailed and specific information, please refer to these guides:
https://support.thingstream.io/hc/en-gb/articles/19114534152604-MQTT-Anywhere
https://support.thingstream.io/hc/en-gb/articles/19115060756892-MQTT-Flex