Turns out that I need to change my SIM card once in a while.
Do do so, it is necessary to remove six screws from the main case plus two screws for the mPCI LTE card. Now, I am looking into ideas how to improve this.
No real progress so far. But I was planning to look into notebook spare parts. Mainly, because the cables are round, so that a simple whole needs to be drilled to the case.