@fm ps. I noticed last few years, that the mosquitto test server you refer to is occasionally not available. Before root-causing-finding your embedded MQTT application, its worth verifying the test server is up and running. I never face any issues with low costs services e.g. cloudmqtt.com

@florian-duecker I’ve revolved the issue by using the off the shelf OpenVPN option T-Mobile offers: Summarized for others&future:
The IoT Easy Connect LTE-M connectivity assings a private IP-range (in my case 10.42.242.0/24) to the sim-cards.
The IoT Easy Connect Portal>Settings>OpenVPN describes how to setup an OpenVPN connection to T-Mobile.
Activating this tunnel on my server enabled me to ping the devices and to carry out various connectivity test.

@fm ps. I noticed last few years, that the mosquitto test server you refer to is occasionally not available. Before root-causing-finding your embedded MQTT application, its worth verifying the test server is up and running. I never face any issues with low costs services e.g. cloudmqtt.com

Is there a way to IPv4 route to a LTE-M subscriber from the internet? (read: can it be exposed to the internet by having a public IPv4 address)
Background: Before going into private APN I would like to validate functionality.
For a particular&limited end2end test I prefer to be able to connect a LTE-M device from the public internet.
Yes, Its (really!) temporarily and I’ll use firewall etc within the device and pse notice that the public internet exposed IPv4 address does not have to be fixed for the test.