You'll want to make sure that you've selected the appropriate permission for your location tracking. This way, you'll have the mileage option in your mobile app.
If you're tracking mileage in IOS, you can go to your phone settings then set the phone's location permissions to "Always." Also, enable the Motion & Fitness permission for the most accurate trip tracking.
However, if you're using an Android, go to your phone settings and set the phone’s location permissions to “Allow all the time." And physical permission to "Allow."