as far as i understand roaming is not implemented for NB-IoT in the DT Network, neither in netherlands nor in germany. you will Need an german SIM from telekom to run your shield here.
By now I have my sodaq shield running with the Cloud of Things by Deutsche Telekom. I runs fine an the CoT is realy a better alternative to All Things Talk.
Unfortunately the Arduino Leonardo has very limited space for code, so I had to do a very small and rougth mqtt-sn implementation, there for I added UDP Support in the sodaq libraries and hat to disable/comment other parts of the sodaq-libraries, that i do not Need to get it fit into the Leonardo.
If anybody is interested in this code (udp-support and rought mqtt-sn) send my an email.