DIAGNOSIS: This is almost always a power or overheating issue. The router is shutting down to protect itself. The internal components, especially the processor under load, generate a fair bit of heat.
FIX: 1) Check the power supply brick and cable. Make sure it's the original one and it's plugged firmly into both the wall and the router. 2) Feel the top and sides of the router. Is it very hot to the touch? 3) Clear the area around it. It needs space for airflow—don't stack things on top of it or cram it in a closed cabinet. 4) Consider pointing a small desk fan near it, or moving it to a cooler location.
IF THAT DIDN'T WORK: The firmware might be corrupted. Try a manual firmware update via the DumaOS interface (Settings > Administration > Firmware Update). If the problem persists, a factory reset (using the small recessed button on the back) can clear out any bad settings, but you'll have to set everything up again.
HEADS UP: Using a cheap power strip or a heavily loaded UPS can cause unstable voltage, which can also trigger reboots. Try plugging the router directly into a wall outlet to rule that out.