DIAGNOSIS That's almost always a power supply issue. The external AC adapter for this router is known to fail, causing intermittent power drops.
FIX First, unplug the power brick from the wall and the router. Check the cable connections at both ends are fully seated. Try plugging it into a different wall outlet directly, not a power strip. If the rebooting continues, you need a replacement adapter. You need a 19V DC, 1.75A, center-positive 5.5x2.5mm barrel plug adapter. An official ASUS ADP-65DW B is the exact one.
IF THAT DIDN'T WORK If a known-good power supply doesn't fix it, excessive heat can cause thermal reboots. Make sure the router is upright (vents on top and bottom clear), not in an enclosed cabinet, and not stacked under other gear. Blow any dust out of the vents.
HEADS UP Don't just buy any 19V adapter—the polarity and plug size must match exactly, or you'll fry the router.