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I think there is a manufactuing error on (at least some of) the shields.
Firstly I also couldnt get it working via GPIO. Then after spending several hours on sofware, schematics and comments, I came up with a solution. You should not have a connection on “pin 20” between Pi and the shield. Pin 20 is GND on Pi schematics. On Shield schematics it is indicated to be a non-connected pin but in reality it is connected. When you press Power*up button, 3.8V is injected to this pin.
To sum up, break the connection between Pi pin 20 and the shield.

PS: I assume you are pressing Power-up button after you plug the power to the shield. Shield do not work until you power it up by the button.