Certified GKH Gardening Expert
The way plants grow is as follows: The tip is the only point that will elongate. The middle of a branch or stem will generally not stretch. This is done by tip growth. Tissue also expands outwards to grow wider, but never lengthwise.
It looks like it was pruned very young. It needed to grow a tall leader before being pruned if you wanted to make it taller. For now, the only way to achieve this will be to choose the straightest branch and cut off all but that one. It will set the whole plant back for a few years, but you could get a taller plant out of it by then.
I don't recommend this. Where is is now is where it needs to stay, other than annual pruning. It is just an option for making a taller shrub.
This article will help you with their care: https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria.htm