For the terminally curious, this is a shot of the BMS board.

If your ROG Ally doesn't power on, check:

Did you install the light blocker on your new back cover? Did you try taping the light sensor over with black tape?

Did you plug your Ally into mains using the original charging brick?

Double check to ensure your cable is properly seated on the mainboard. The margins are tight and it can pop out easily.

Double check that the pins on the connector head are inserted fully. You should be able to see the copper tabs through the white plastic.

Check your wiring. With the connector heads little white tabs facing up, you should have 3 red wires on the right and 3 black wires on the left. When connected to the mainboard, you should have 3 red wires on the left and 3 black wires on the right.