One solution would be to use monitoring to have your app relaunched if it’s killed and a beacon event (enter/exit) comes. And then, once your app gets relaunched into the background, you could range for a few seconds (no special work required), for up to 3 minutes (starting a background execution task required), or indefinitely (starting regular location updates required—you need a really good reason for that though, otherwise you’ll have your app rejected from the App Store!).
What app are you building? Why does it need to constantly range even if the user is not using it? Keep in mind this will drain the phone’s battery, as constant ranging is not as energy efficient as simple enter/exit monitoring.