AR750S reboots itself and loses settings?

I’ve had a GL-AR750S for awhile (obviously, it being discontinued) and it has worked fine. I used custom firewall rules to set TTL to 65, hoping Visible won’t limit speed.

Recently it updated itself to version 4 (I think), the LuCi interface for which is different from what I’m familiar with. In the process, it lost previous settings such as SSID and WiFi password.

I downloaded and reinstalled firmware 3.216, disabled auto-update and restored previous settings. That worked but the router continues to repeatedly reset itself. It happens only when it has no power.

Can someone suggest why it happens at all? It didn’t do that before.