S23 ultra lcd change, no signal fault

I have just replaced a screen on a s23 ultra with a Samsung service pack screen. The phone now has no signal and nothing was damaged in the transfer as I am always very careful during repairs, this has happened recently on a s20 ultra with exactly the same result. I have been fixing phones for the last 20 years and have never had an issue like this before. Does anyone know if this model can develop a fault like this just by switching the motherboard to another screen? I’m scratching my head here and just wondered if anyone has had this problem before on this repair. Changed charging port and tested motherboard back in the original screen and still no signal? Any replies would be appreciated.

Thanks