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There may be different network modes available depending on where you are. The network mode influences the data speeds your phone can achieve.
1. Find "Mobile Data Options"
Press Mobile Data.
Press Mobile Data Options.
2. Select network mode
Press the indicator next to "Enable 4G" to turn use of 4G on or off.
If you turn on the function, your phone automatically switches between 4G and 3G provided you're within range of a 4G network.
3. Return to the home screen
Press the Home key to return to the home screen.