Do you know the history of the device? If it was dropped, repaired, came into contact with water or if the customer was using cheap chargers, then that would help you in your troubleshooting.
I agree with A1, try isolating the logic board with a known-good battery and Lightning Port. If it still doesn’t boot, then you definitely have a board level issue. Depending on the history, you could focus on the charge circuit, or probe the PMIC/BBPMU and main voltage rails for additional clues.