Solution - How to fix the issue that WiFi disconnected when iPhone is locked?

I had this problem with my iPhone 8 for two weeks. I had tried all means available on the internet which used to work on some people's iPhones (https://www.imobie.com/support/wifi-disconnects-when-iphone-is-locked.htm) but the issue on my iPhone persisted. I also called Apple Support twice and they couldn't help much but asked me to upgrade the OS from 12.4.1 to 14.2 which I don't want to. I also turned off battery performance checking because my battery had only 78% capacity so that iPhone wouldn't do any suspicious actions on the backend because of the poor battery.

Occasionally, the issue got resolved and I want to share my experience and hope it works for other people too:

Unlock the iPhone > Connect to WiFi > Go to App Store > Update all apps > Lock your iPhone

You know iPhone has a switch that disables download/upgrade when the size is bigger than 200MB. I think this might trigger/reset that switch back to normal.

Please share with us if it's working for you too so we can help more people.