When you insert two SIMs into your phone, you can make calls and send and receive messages from both SIMs. You can also select a default SIM for mobile data usage.
Slide two fingers downwards starting from the top of the screen.
Press the settings icon.
Press Network & Internet.
Press SIM cards.
Press the indicator next to the required SIM to turn use of the SIM on or off.
Press the required SIM.
If you select the SIM, which isn't the default SIM for mobile data, press OK.
Press the Return key.
Press Preferred SIM for unknown number.
Press the required setting.
Press the Home key to return to the home screen.