First, I would suggest to update your device's firmware to the latest to see if your issue has not been solved already. The firmware file can be downloaded here. Before updating, disable Keep settings option.
What are you using as your MODBUS master? I had no issues testing with QModMaster.
Also, if you were to look at the parameters available to get via MODBUS, provided here, different parameters can be represented in different formats, some numerical, some text. Such parameters as, for example, System Hostname, do not change over time.
Since the first 20 registers cover some mobile related data, is your device connected to some mobile operator?