I Need Advice?

I have changed potting mixes,greenhouse or outside,cut off will reroot, root stem does nothing

Wow. That looks pretty sad. Did something chew on it? I hope somebody can give you some good advice.

No,It's right at the top of the soil line in pot.

Cut that off. The bottom part has roots and will grow, and eventually sprout new branches. The top part needs to be rerooted.
This happened to some other people and there is still no clear explanation what causes this.

It almost looks like what they call damp off it happen with seedlings all the time. With a little luck you will have 2 now one short and one longer check out the damp off

It's desiccation caused by keeping the plant too dry. It dries up at the soil line. This can also happen when people put pea gravel on the top against the plumeria. If there are nodes beneath the dried up part, it will sprout new branches. If there aren't, it will do nothing and eventually die. You can cut above the dried part and re-root it or graft it.