We have a new board PI 3B+. It works fine when it is standalone, powered by USB power supply. We try to power on by GPIO pin n. 2, 4 (+5V) and GND on pins 6, 9, 14, 20, 25, 30, 34, 39. In this case the board not work. It turn on its red LED POWER OK, but the green LED ACT is always turned off.
Important note: before the new PI 3B+ we have used without problems the same method with the PI 3B.
Could help us? Thanks.