NAS devices are notoriously inadequate Time Machine storage devices. You might happily bebop along seeing it dutifully back up, but then when it comes time to restore, it doesn't work.
They didn't work because they used a hack of Apple Filing Protocol (AFP) to qualify as a Time Machine destination. While the hack worked for regular file sharing, it failed at hosting a Time Machine destination. Time Machine was never supported when using a NAS that used that hack.
There is a new, supported specification for Time Machine over SMB, so that might provide a functional backup to a NAS.