Slide your finger downwards starting from the top of the screen.
Tap All settings.
Tap Network & wireless.
Tap Mobile & SIM.
Tap SIM settings.
Select network mode
Tap the drop down list below "Highest connection speed".
Tap the required network mode.
There may be different network modes available depending on where you are. Depending on the network mode selected, your mobile phone will always find the network with the highest data speed and best signal.