Routeros and firmware match. Full hardware freeze, even the ports go hard down.
Shame the device does not have a serial port to see what happens at the time of the freeze, do you see if there is any auto-supouts generated from around this time and what does the log say the reason for shutdown/reboot is?
Do you notice any patterns yet with the reboot, i.e if there is X amount of days before the devices reboot etc? As well as this I know there had been some issues with romon in the past causing random lockups like this, do you happen to have this enabled currently and if so can you disable it for a while and see if this might fix the issue?
Lastly, how are you powering the device, are you using the supplied PSU?