I have been using certain cloud backup service for a few years now. It is not perfect, but they have a decent desktop client and mobile app. And there was a due date for my annual subscription payment.
CrashPlan will no longer be free for users who wish to use it for local backup. It was a great backup tool with continuous backup, automatic cleanup of older backups and simple to use interface.
Just a though – instead of buying a NAS drive get a cloud backup subscription.