Thingy91 - start-up problem will not connect to LTE - please help :-)

sorry for the perhaps stupid question - but despite several hours of intensive searching, I am not getting any further with the start -up of my new and beautiful Thingy91.
Unpacked and proceeded exactly according to the get started video ( -   Updating Zip file 2021-03-04  downloaded  - Modem firmware updated and Asset Tracker - Hex (thingy91_ltem_2021-03-04_8e8c6089.hex) for LTE installed. Everything worked.
SIM card activated and now the problem - no LTE connection - even though I am trying for hours in the middle of Germany with a well-developed LTE network. The LTE Link Monitor shows Connecting to LTE network - but then LTE could not be established .. Question: was that the correct hex file for the LTE asset tracker? Or where is my mistake?
Thanks in advance and stay healthy