URGENT: I replaced a battery, and now fails to boot.

Hi Crew.

I got an iphone 8 in for repair, and it seemed pretty straight forward. Replace the battery.

Cool. started by disconnecting battery from the board. I heated up the back a little bit to loosen the adhesive and pried up the batter.

I installed the new battery, and it booted up fine while the screen was off the housing. everything worked fine, so it was time to close it up. I closed the device, and it stopped responding. screen went black, and I wasn't able to boot or wake it. I lifted the screen, disconnected the battery, and reconnected it. the device worked flawlessly again for 2 minutes. so i closed the device, and again, it stopped responding.

I opened it up, put the old battery back in, and it wouldnt even turn on at all.

i took the battery to the load tester, and it reads fine, charges fine. now with both batteries, the backlight doesnt turn on, and i only get about 5 seconds of apple logo with no backlight, then it shuts off again.

i tried replacing the screen, thinking maybe i tore a flex, but that didnt fix the issue. the device is now stuck in that mode, and i dont know what to do. our customer is rightfully impatient for their device, as they were expecting a 20minute battery change.

any help asap would be appreciated.