Signal metrics are indeed on the worse side, specifically SINR value, which during file generation, was -5 dB. It is a guaranteed disconnect, however, it might have been just a momentary hickup.
To determine, if the issue could be caused by poor signal quality, in router's WebUI Status -> Network -> Mobile you can monitor signal values. For reference, look up this page.
By "I do not have a PING reboot function" do you mean that there is no Services -> Auto reboot -> Ping reboot page?
Also, could you update your device firmware to the latest 7.04, which you can download here with Keep settings option set to off and check, if the issue persists? Be aware, that this will delete your configuration.
Lastly, could you test connection quality using different LTE bands? Instructions on how to lock different bands are provided in this video. The bands to try are: B1, B3, B7 and B8.