I did narrow it down due to the network-manager package. I disabled the network-manager service and configured my Sierra Wireless LTE 4G module (which is connected through mPCI to USB) with the MBIM tools and now it works again as expected. With the network-manager, nothing worked, after 1 minute the nm-network interface gone, disconnecting/connecting did not work. But hey, I got it to work by doing something else, so I am happy, however the configuration is a little bit more tedious. I suppose I will use chat gpt to make me a bash script for it 😉