iPhone 8 Plus won't turn on, charge or show up on MacBook/pc

Reciently a family member gave me an old iPhone 8 Plus to repair. The back glass was shattered, the screen was cracked, it wouldn’t charge but would sometimes turn on.

I removed the back glass using a heat gun and prying tool (I don’t think I used enough heat to damage the components - I could touch the parts that were heated up without it being uncomfortable or burning my fingers). After removing the back glass the phone turned on. I replaced the charging port board, battery and screen. Now I can’t get the phone to turn on at all, nor will it charge or show up on my Mac/PC.

Just to be safe I took the other components out and checked them for heat damage but couldn’t find any. All the connections are secure. I have tried using different cables which work with other iPhones, tried leaving it to charge for almost a full day and tried doing a hard reset, none of these options worked.

Any assistance would be apprecieated.