all of our SIMs for M2M and IoT are G-SIMs, which means that technically they are always roaming. So a query for the registration state, for instance with AT+CEREG, should always yield: 5.
On our testcards, we have currently no HPLMN deployed. For this reason the modem searches for a suitable network and will consider any NB-IoT network on the supported bands.
To make sure that the modem will only connect to the DT network, you can either fall back to manual network selection (PLMN-Id=26201), or restrict the supported bands to band 8. This will exclude any operators in Germany for which we have not yet roaming agreements.