You can set your tablet to select a network automatically or you can select a network manually. If you select a network manually, your tablet will lose network connection when the selected network is out of reach.
Press Settings.
Press Wireless & networks.
Press Mobile network.
Press Carrier.
Press the indicator next to "Choose automatically" to turn off the function.
Press OK and your tablet will search for networks.
Press the required network.
Press the indicator next to "Choose automatically" to turn on the function.
Press the Home key to return to the home screen.