Restore iPhone blocked in Demo Mode

[Auties00]

Hello, I recently bought a pile of used broken iphones. After fixing one of them, I discovered that this iPhone was once a Demo device as it displays the message "This iPhone is property of an Apple Authorized Reseller and is not for sale". I checked the IMEI of the iphone and it isn't flagged as stolen, blocked or locked and so it can be activated . So I decided to reset the iPhone through iTunes, but on the second loading screen(https://support.apple.com/en-us/102317), the phone stops the reset process and returns to the screen I mentioned at the beginning of this post. I'm pretty sure this is a local issue as the iPhone isn't connected to the internet and even if it were, the imei is clean so it should be able to reset. After trying itunes, I tried to use idevicerestore, but it ended just as it did with iTunes. I was wondering then if there's any way to forcefully reset the phone using this library. My only other option is to call Apple and convince them to unenroll the phone from Demo mode, but I'm pretty sure that would be hard to do, so I figured I might as well ask.