Thought I had this persistent issue with the AX88u on B4b shutting down in the middle of the night and rebooting.
Troubleshooting the issue for a week. Thought the router might be overheating, because when I have a small tower fan running and blowing air on it, there is no issue. Looked through the logs, nothing. Always about 3am, down it would go, and by 3:30am it was back on. Yesterday, while on the PC, I take the headphones off and turn off the fan for a little quiet time. At that exact moment one of the wife's cats jumps on the desk, goes behind the second monitor, lies down, flipping the power switch off on the power strip that's behind it. Router, Modem and NAS all down. A few seconds later as it moves around to get comfortable, it flips the switch back on
Solution: New power strip with a protected switch (harder to accidently flip). Also moved it behind the NAS to better protect it. The power strip now protects the UPS and everything plugs in to the UPS. The fan runs overnight (to keep the cat away, not cooling) and the door to the air conditioned office is closed and the door knob changed so it can't open the door on its own.
3 days solid, so far so good