Why does my iPhone 4's Wi-Fi keep disconnecting when I lock it?

My iPhone has poor Wi-Fi, because my brother threw it and it has always had poor Wi-Fi ever since. He fixed a little bit but now it disconnects Wi-Fi when it locks, it doesn't disconnect right away, maybe a couple seconds after you lock it. I researched about it and it said that it does that to save battery, It never did that before until today. I also don't get notifications on instagram or insta message or Kik. Even when I have Wi-Fi, so please help me with both problems.

Not sure about the messenger query but Wi-Fi always disconnects when your phone is locked and reconnects once you unlock it. That isn't out of the ordinary.

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