So, the backup provided by the vendor will not restore if the destination is backend storage. But it will restore into SQL Server if you use the NAS storage.

SQL Server can see the NAS storage obviously as a valid storage path. But does it see the backend storage as a valid storage path?

Since you restored to the NAS storage, are you able to take a backup of that database and restore that backup (not the vendor supplied version of the backup) to the backend storage path?

Are other databases currently accessing the same backend storage (to which you are trying to restore) from this SQL instance?