There may be different network modes available depending on where you are. The network mode influences the data speeds your mobile phone can achieve.
Slide two fingers downwards starting from the top the screen.
Tap the settings icon.
Tap Network & Internet.
Tap Mobile network.
Tap Preferred network type.
Tap the required network mode.
There may be different network modes available depending on where you are. If you set your mobile phone to the fastest network mode, it will always find the network with the highest data speed and best signal.