You can find the corresponding option in the Reagle App: Lock setting tab->Edit->Auto-unlock. Make sure to set the correct location of your lock on the map so that auto-unlocking can work properly. One also needs set the detecting radius after auto-unlock is enabled. Once you get inside the range you set, the Reagle app will start the unlocking attempt in the background. However, the lock will not be unlocked until you walk inside the Bluetooth range of the lock.
Note that the auto-unlock performance will be affected by the accuracy of your location info on your phone and removing the Reagle app from the background will also reset the auto-unlocking attempt.