Tried to tighten the brakes, won't grip at all
On the first picture, if you look closely at the cable right to the bolt at the center, you will see deformation marks from the previous clamping. The position where you have clamped the cable seems to be too slack. That is, too much cable is left between the lever and the pinch bolt.

To fix it, loose the cable, press the "thing" that rotates when you press the brake lever clockwise, and clamp the cable again at the new place.

You may end up taking up too much slack, permanently pressing the brake pads against the rotor or drum; in this case, find a compromise position between too slack and too tight. It can be done by using the adjustment barrel at the lever, if you have one there.