Dell Inspiron 5521 Factory Reset

Hello, I bought a Dell Inspiron 5521 laptop in 2013. Now I would like to transfer it to a new user and I would like to to reset it to its factory status.

Things I have tried so far:

- Factory Reset in BIOS setup. This option is disabled, I cannot select it.

- Dell Backup and Recovery. I think I uninstalled this application years ago because it was annoying me all the time with messages to make backups of my computer to some kind of Dell cloud, which I didn't want to do. I downloaded it from dell.com and tried to install it, but the latest version is only for windows 8 and in the meantime my laptop was updated to windows 10. I get a message that the software cannot be installed because it is only for dell devices.

- Windows 10 Factory Reset. This is the factory reset available through the windows settings. After a reboot and a some time looking to the screen I get this message: "Problem beim Zurücksetzens des PCs. Es wurden keine Ändereungen vorgenommen" (english: problem by reset of the PC. No changes were performed).

- Windows Recovery Image. I searched in dell.com/drivers and there is no image available to download.

Can anybody help?

Thanks