The purpose of a weightlifting belt is to boost the effect of a valsalva maneuver in increasing intrathoracic pressure during heavy lifts, where that pressure is used to keep the torso rigid. You take a deep breath and hold it in against a closed throat, and then push your abs out, against the belt to increase internal pressure. The role of the belt is to hold abdomen in so that the pressure is maintained rather than dissipating as it pushes the abdomen out. Whereas without the belt, you can only muster as much internal pressure as your abs can hold in.
Holding a plank is very different to heavy lifting, in that extended isometric contraction is required for often up to several minutes, whereas in weight training, each rep only takes a few seconds, and the lifter can relax between reps, such that after each rep of exercise, the lifter releases their breath and then performs a new valsalva maneuver for the next rep. As you cannot possibly hope to hold a valsalva maneuver (requiring a breath hold) for anywhere near the duration of a plank, I would suggest that a belt would probably not be beneficial. Instead I would suggest that you focus on other methods of strengthening your abs until they are strong enough to be able to hold a plank without letting your abdomen sag. Either that only do very short duration planks or planks from the knees with someone watching and either cueing (verbal reminding) you to straighten your back whenever it sags, or to only do the exercise for whatever short amount of time you can manage without relaxing your spine.