Actually it is possible to get RSSI information from the Teltonika routers WITHOUT logging in, if you tick the "Show mobile info at login page" selection in the setup. There is also a python3 script to handle this easily, available at https://extinfo.uniscada.eu/support/teltonika_signal.py
>>> from teltonika_signal import TeltonikaSignal
>>> t = TeltonikaSignal()
>>> t.get_teltonika_data()
{'operator': 'Telia Telia', 'connection_type': '4G (LTE)', 'signal_strength': '-67 dBm'}
>>>
There is a limitation however - it works with older firmware only, RUT2XX_R_00.01.x for example.
The latest firmware (like RUT2_R_00.07.x) does not seem to have this information available before logging in, unfortunately.