Hello,
To start with, could you check full mobile signal strength quality aspects? I.e. there are multiple values, which can describe mobile signal quality. 75 dB RSSI value which you just indicated is just one of them. From "Status -> Network -> Mobile" menu could you also check RSRP, RSRQ and SINR values?
Could you also share more details what happens when issue occurs? Assuming auto reboot would be disabled, would router not recover and remain without internet connection for long time? Is it possible that you simply have auto reboot configured on the router for 4 hours 15 minutes period, which is causing your periodic disconnections?
Could you wait until issue will appear again, then download troubleshoot package from our router and share it here? (Troubleshoot package can be downloaded from "System -> Administration -> Troubleshoot" menu)
Note: Troubleshoot package contains router's configuration information and current router's logs. All passwords and certificate information is hidden automatically (not included in troubleshoot package). Nonetheless, it is recommended to remove any other information (e.g. phone numbers) that you might not want to share publicly.