LB2120 no 4G connection after one month of use

I bought the LB2120 Modem to connect to the internet over 4G with a Telefonica/O2 SIM in Germany. I checked 4G availability before buying with my phone and the Modem worked well for the first month of the contract until last Sunday. Instead of 4G / LTE the device only connects with GSM and low throughput. Neiter reboot, reset or any other ideas from the forum did any good. I also removed the SIM card from the device and checked with a smartphone where the SIM card works in 4G / LTE mode.

There is not much information provided by the device either.