DIAGNOSIS Faulty power supply or a scheduled reboot feature is enabled. Overheating can also cause this.
FIX Check for a reboot schedule. Log in, go to Advanced > System Tools > Time Settings > Reboot. Disable "Enable Reboot" if it's on. Check the power adapter connection at both ends. Ensure the plug is fully seated in the router's DC-IN jack.
IF THAT DIDN'T WORK Update the firmware. Go to Advanced > System Tools > Firmware Upgrade. Check for update. If none, perform a manual factory reset using the button. Set it up fresh without restoring old settings.
HEADS UP If the router reboots during high network load, that points to a failing internal power regulator. Contact TP-Link support if under warranty.