These three events are very useful when working with LWM2M devices, because these devices sends an update every some time to the LWM2M server.

Use this body with the Create subscription request. Type must be set to "lifecycleEvents"

  'subscriptionType': 'lifecycleEvents', 
  'groupName': 'group_name', 
  'deletionPolicy': 0, 
  'events': ['registration', 'deregistration', 'update'], 
  'criteria': {
    'serialNumbers': ['<imei>']

In this way you will be able to see the registration and the update and the de-registration of your lwm2m device.