Set a Wi-Fi disconnect policy
By default, the tablet disconnects from Wi-Fi when the screen turns off and connects to a mobile data network instead. It does this to conserve battery power, and reconnects to Wi-Fi again when you wake it.
You can change this policy to stay connected to Wi-Fi when the tablet is connected to a charger (when battery life isn't a problem), or never to disconnect from Wi-Fi, which trades battery life for reduced mobile network data use.
1.On the Wi-Fi settings screen, touch Wi-Fi disconnect policy.
2.Touch the policy you want.