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 Voice & Data.
To use 4G for both voice calls and mobile data, press 4G, VoLTE On.
To use 4G for mobile data only, press 4G, VoLTE Off.
To turn off use of 4G, press 3G.
Slide your finger upwards starting from the bottom of the screen to return to the home screen.