I found when my device is on 'Alarm only' its LED light dosent work, when i changed it to 'do not disturb' i saw it works. now i want to know how i can use it in 'Alarms only' mode?
I assume you are talking about the Do not disturb setting when you say "alarm only mode" If so go in Settings, Sound & Notification, Do not disturb, Priority only allows and set what you want to receive while in Do not disturb mode.