Because of a firmware bug in the pre-installed firmware 1.00 (1.00b03) of the DWR-921 Rev.C3 the SIM-cards won't be recognized correctly when the SIM-card is secured by PIN.

Please install the actual firmeware this you can download from
http://www.dlink.com/nl/nl/support/product/dwr-921-4g-lte-router?revision=de_revc3#downloads

Optional disable the PIN on the SIM-card. For this please use a mobile phone, tablet or internet-stick.
After this plugg-in the SIM-card into the DWR-921, remove the PIN from the DWR-921 internet connection setup, save this settings and restart the DWR-921 with his power-off/on switch.