Wifi channel changes at 4 am?

I've set my WiFi 5G channel to 165, 20 Hz, which is unoccupied by anybody else. I've turned off DFS and auto-channel. But when using Wi-Fi analyzer or NetSpot, I can see clearly that the channel changes on its own every day at 4 AM, not before or later. But the Fizz Wi-FI app of the modem pages still tell me it's own 165 when it's not.
When it happens I can either reboot the modem, or switch to another channel and come back to have it go to the correct channel.
What happens at 4 AM and how do we prevent that from happening?
I have put a mechanical timer that turns off the power outlet after 4 am and that works, but that's so clumsy to have to turn off the modem for 30 minutes just to have it keep its channel!
Please fix this! I've been complaining for months and some other users reported the wi-fi channel changing on its own too!