Screen won't rotate out of portrait mode -

I landed on an iPhone 5 (GSM) that has a problem with rotating the screen. It won't get out of portrait mode.

1) Portrait mode lock is NOT on. Tested in Notes and Safari

2) Restored to iOS 7.1 using itunes. Did not fix the problem.

I'm not sure if this device has been tampered with an possibly an interconnect cable or something was not connected properly?

If someone can advise what part would need to be changed, it would be greatly appreciated. I have no idea where to start with this one and I don't want to buy an expensive LCD/Digi assembly if it's not going to fix the problem.