Time machine doesn't recognise my hard disk connected to my Netgear router

My Time machine suddenly not seeing the hard disk connected to my Netgear router.  A littl eover a year ago I have purchased the Netgear R7000 router and connected an external hard disk to it and shared out so that my Time machine can use it for backup my Mac. All in a sudden, my Time machine tells me that the backup failed due to couldn't find the backup drive. I then tried to mount the drive on my mac and have successfully mounted and see all the files on the hard disk. I then open up Time machine and try to select this hard disk again as the backup disk but it doesn't come up in the available backup disk list. Had anyone encountered this issue before?