As you can see in the image below the forearm is able to twist ok on the Y axis, but the hand can't turn up or down as it also bends on the Y axis. Is there a way to fix this without using shape keys or drivers? Because i don't know python code.
By disabling both of your constraints (eye icons) you can get full control over the hand. You can even just disable one of them and see if it makes you the desired change.
You can also play with the values of the constraints if you want to keep some limitations to the bone.