8+ Old screen works (kind of) new one doesn't

My iPhone 8+ suddenly only showed an unstable half screen (vertically) so I could only see the right side. Bought a new one from iFixit. I have experience replacing these so I have some confidence I am doing it correctly.

The new one is DOA. Put the old one back and it works but with the same problem as before. Put the new one in for a second time, nothing. Put the old one back in again and it works as before.

Did the volume up/down power trick and held for two minutes — nothing. Any ideas?

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The phone is on but the screen is still black. I know this because find my phone plays the phone. iTunes does not find the phone.

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