Why does an iPhone 7 not turn on with charging port plugged in

Hello, I had an iPhone 7 come in DOA. No signs of life, cracked screen, wouldn't take a charge. After opening the phone, it has no signs of water damage or anything visibly broken. I went through steps of diagnosing the issue. Started with new screen (no fix). Then connected a different battery, nothing. Then decided to replace the charging port. At this point the phone came on, but went into a boot loop. So I disconnected the charging port and reconnected the second battery, and the phone came right on with the original cracked screen. The replacement charging port is an OEM charging port. I am assuming the original battery that is still connected is just very dead as it obviously wouldn't take a charge from the original charging port. Any ideas?