Not sure if this has been posted here before, but you can find out what cell carrier your XC40 P8 is using for its 4-year "free" Google data plan. In my case here in the US it's T-Mobile, which I believe is the default carrier for "Google Fi" plans.
If you go to Google Play, login with your Google account, and click on the Settings (gear) icon, it should show your car as one of the Play Store devices:
If you go to Google Play, login with your Google account, and click on the Settings (gear) icon, it should show your car as one of the Play Store devices: