There may be different network modes available depending on where you are. The network mode influences the data speeds your phone can achieve.
Press Settings.
Press Mobile Data.
Press Mobile Data Options.
Press Voice & Data.
To turn on use of 5G primarily, press 5G On.
To turn on automatic switch between 5G and 4G, press 5G Auto.
To use 4G only, press 4G.
Press the Home key to return to the home screen.