There may be different network modes available depending on where you are. The network mode influences the data speeds your phone can achieve.
Press Mobile Data.
Press Mobile Data Options.
Press Enable 4G.
To turn off use of 4G, press Off.
To use 4G for both voice calls and mobile data, press Voice & Data.
To use 4G for mobile data only, press Data Only.
Slide your finger upwards starting from the bottom of the screen to return to the home screen.