Application Run In Background

So I could use some help, I'm creating a Timer application and need it to run in the background when then screen is locked. I've tried using Application.RunInBackground to no avail, I've tried figuring out the Player Settings with 'Behavior in Background' custom/suspend/exit options and nothing seems to work. Does anyone have suggestions or plugins available that could solve this problem?

Basically, the app is a workout timer app and when the phone screen locks, the app suspends. I need the timer to run when the app is locked so it can alert the user that their workout timer is up.

Any help would be much appreciated.