Do you have any insight as why the phone died; drop, previous repair attempt…water? That would give us a clue as to where you start.
Try booting just the board, known-good battery and Lightning port. Then connect it to an iTunes enabled computer. If it is recognized, then some peripheral is causing issues. If it isn’t, then you likely have a logic board issue. You could start probing the board by looking at the main voltage rails (PP_BATT_VCC, PP_VCC_MAIN and PP5V0_USB). If you’re not comfortable with that, have it looked at by a micro-solderer.