Wifi issues iPhone 12 Pro Max

Sorry, I am not a mind reader, and you didn’t say what troubleshooting you have already done. Maybe you’ve tried the suggestions below, as you didn’t say whether you had or not.

Next step is to forget the network, restart your phone, and reconnect.

And after that, Reset Network Settings on the phone.

And see→Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support

And how far is the phone from the router, compared to other devices?

And if none of the above fix it use the Get Support link at the top of this page to arrange for Apple to analyze it.

So when you go to settings and wifi does it say that you are connected to the wifi? If you go to wifi settings try click on the little blue (i) icon, then scroll down to where it says DNS (Configure DNS) what does it say? Set it to automatic unless it already is. Same for configure IP, set this to automatic also if it isnt already.

Then check if you can open a webpage or something in safari, if still doesnt work then I dont know what it could be.

I see Lawrence gave you some tips you can try also.

Please define “stop working”. iPhones disconnect from Wi-Fi and turn off Wi-Fi 30 seconds after the screen locks, to preserve the battery, unless the phone is plugged in. Is that what you are observing? If it isn’t describe the scenario where it stops working.

True except if you have something running like an app that uses data, like if you are streaming something from your phone to your TV, then it wont disconnect from wifi because you need the wifi even after the screen has locked.

That is correct, if it was actively using Wi-Fi when the screen locks.

There are other exceptions; if the phone receives a notification over cellular data Wi-Fi turns on briefly to allow the app that is the target of the notification to file and display it.

But I was keeping it simple for a first pass answer.

Thanks for your input, however.

First troubleshooting step is to restart your router. Even if 1,000 other devices are working OK.