What am I doing wrong with this dummy battery for my camera?
The Canon LPE12 battery has four terminals.

Two are power.  One of the others is probably a temperature sensor for the charger.
The fourth terminal is usually some kind of identification pin.  It may be as simple as a resistor indicating the capacity of the battery, or it may be as complicated as some kind of encrypted digital query from the camera to the battery.
You have to figure out what the other two pins do, and emulate those functions in your battery grip.
The camera probably checks both of the extra pins and won't work if either gives an incorrect result.