According to Airtel website the process for iOS and Android devices would differ a bit. Go to the settings tab Get to Connections or Mobile Network Select the 5G network mode. You may need to update your handset software to do the same. By default, it can happen that your new 5G smartphone might also be running on the 4G setting only. If you want to get 5G services, then ensure that your smartphone's 5G connection is active.