Anytime that you make a backup, always check to make sure that the storage is going to the external drive or sdcard1. Backups mostly by default use the sdcard0 which is internal storage, which if you do a factory reset then the backup is gone. What I would suggest with your device is go into Settings> go to Developer Options> turn it on and check the check box for USB Debugging. Then plug in your device via usb and see if you can search for the backup on your computer.
One other thing, I'm not sure what you backed up, but if it is apps that you had and want back. One thing you could do is go into the Playstore, click on 3-dot line in the upper right corner of the screen and select "My apps", go into "My apps" which displays what is currently installed, slide the screen to the right and you will see "All", this stores every app that you've ever downloaded, if you are concerned about apps and a backup of apps that doesn't exist, this could be a good way to get those apps back, but keep in mind, the Playstore only keeps track of the apps, it does not keep track of the data, like passwords or settings, you'll need to restore that information.
