This may be a shot in the dark but make sure the phone is up to date. I have had one case where aftermarket digitizer on a refurbished iPad Air 2 touch screen not work because the iOS was too old like iOS 10 / 11.
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What quality is the demo screen? Some may not support writing new display data properly to their display chip.
This may be a shot in the dark but make sure the phone is up to date. I have had one case where aftermarket digitizer on a refurbished iPad Air 2 touch screen not work because the iOS was too old like iOS 10 / 11.