The lower part of the body collapsing while doing plank is common.
Stabilizing the hip and the thoracic spine will allow unloading the aimed area in between - the lumbar spine.
Bracing the core, especially transversus abdominis, and firing the quadriceps muscles is what we want to achieve.
This is a keen reminder that planks are not an easy exercise. They require activation of many different muscles. Feet, calves, thighs, buttocks, abdominals, spinal erectors. All the way up to the shoulders.
If planks feel too easy, maybe you are doing them wrong :)