Wi-Fi keeps shutting off automatically on iPhone?

I recently purchased an iPhone 4S and ever since I connected it to my Wi-Fi home network it has been disconnecting off of the Wi-Fi and using my 3G. The Wi-Fi works great on my other devices, and I have to keep manually going to settings to connect it. Please tell me what to do to fix this issue and let it stay on the network until I manually disconnect it. 10 POINTS for the correct answer

Prob bad connection or you need to reset the router

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