Contradictory messages about restoring to a spare device
I don't think any of the sides are "lying" (i.e. being deliberately deceitful). I think the person wrong here might simply be you.

You're saying that your iPhone 4S died, and you're trying to restore to an identicate spare device. I have a suspicion that your identical spare device is not in fact identical. I think it's an iPhone 4.

The iPhone 4 cannot be updated to iOS versions newer than 7.1.2. In contrast the iPhone 4S can be updated to iOS 9.3.6.

In your case you will need to find newer hardware to restore your backup to.