When you have two
nano SIM cards inserted on your phone, only one card can be used to connect to the Internet at a time.
If you're using
Google or third-party apps, you can also choose which one to use as the default card for making phone calls and sending SMS/MMS messages.
Home screen, swipe up and then tap
Data network, and then choose the card to use for connecting to the Internet. .
Voice calls and
SMS messages respectively to set the default card to use for making phone calls and sending SMS/MMS messages.