Configuration for - UDP over NB-IoT

Dear community

I'm new to the nRF9160 and to cellular connectivity related functionality.

We're in the concept phase of a product that implements cellular cloud connectivity. The solution will finally be lauched in several countries all over the world, and therefore must be capable of communicating via LTE-M and NB-IoT.

I managed to install the "nRF9160: UDP" example and to successfully connect to a UDP custom server and to send some dummy data (via LTE-M). That's already a great start, but now, I would like to achieve the same result based on NB-IoT transport. So far I didn't find the right configuration to achieve this.

Here's my setup:

In order to switch to NB-IoT I slightly changed the prj.conf as follows (the example doesn't mention any config for NB-IoT mode):

CONFIG_LTE_NETWORK_MODE_LTE_M=n
CONFIG_LTE_NETWORK_MODE_NBIOT=y

However, with this config change the system seems not able to establish a stable connection anymore (although it looks like initially a connection gets established).

The sample software produces the following output:

*** Booting Zephyr OS build 3afbbacd2ea2  ***
UDP sample has started
LTE cell changed: Cell ID: 16971977, Tracking area: 600
RRC mode: Connected
RRC mode: Idle

LTE cell changed: Cell ID: -1, Tracking area: -1
LTE cell changed: Cell ID: 2643304, Tracking area: 21110
RRC mode: Connected
RRC mode: Idle

LTE cell changed: Cell ID: -1, Tracking area: -1

(here the connection attempt seems to stuck...)

I would be happy for any hint's (or maybe examples) on how I need to tweak the configuration to successfully communicate via NB-IoT

Thanks a lot in advance :-)