If you haven’t got a known good housing to test the logic board you will need to start eliminating all parts not just the front flex assembly. Try disconnecting all the flex cables besides the screen, battery and power flex. Try booting the phone if it still will not boot connect the lightning port assembly and remove the power flex. Try booting by inserting the charge cable. If it’s still not booting you will need to rule out the battery before moving on to the logic board.