Hello,
Firstly, with signal values like that, it’s surprising that you get a stable connection or any connection at all. Could you please specify what kind of antennas you are using and describe your setup? As I understand it, you are using a camper van setup, correct?
Furthermore, comparing the Rut955 with the RUTX50 seems a bit unfair because the RUTX50 is much more recent and capable. Also, the Rut955 is an LTE Cat4 device and does not support carrier aggregation.
The speeds mentioned in the documentation are theoretical and can only be achieved under perfect conditions, which your router setup is currently far from.
Best regards,
Marijus