My IPhone 7 Plus keep on disconnecting from WiFi

iPhones have always disconnected from Wi-Fi 30 seconds after the phone locks to preserve battery life. For networks that you use you need to go to Settings/Wi-Fi and tap on the i next to the network name and turn on auto-join. If it is already on try turning it off, then on again. And if that fails try restarting your phone-->Restart your iPhone, iPad, or iPod touch - Apple Support.

If that doesn't fix it go to Settings/General/Reset -Reset Network Settings. This will erase your Wi-Fi passwords and restart the phone, but you won't lose any data.

Does this happen with all Wi-Fi networks, or one specific network (such as your home network)?

Does it happen when you are using the phone, or when it goes to sleep (where this action is normal)?