I feel guilty talking about plumerias on a mainly north american forum - - but here are some stories from where they grow easily in the ground

we've collected about six plants - all picked for their colour by my wife - when she first started taking the dog for a walk she'd stop at which ever tree she liked, talked to the owner and snap off a foot or two and stick them in the ground. All of them are growing - we've got a very deep dark pink, one known locally as a fruit salad, a white with yellow centre, a lighter pink and one other I'm not sure of the colour.

One plant we collected - the neighbours said we could take the whole thing - it was about 1 1/2 - 2 metres tall and had been in that spot for ten years and not grown. When I dug it out the root system consisted of one short piece of root and nothing else. That was about five months ago.
It was planted on the nature strip until the recent storms - we've had a bit of flooding and the excess water looked as though it was killing a Himalayan Magnolia (champaca) - so the champaca was moved to the spot the plumeria was in and the plumeria was roughly dug/pulled up with a lovely root ball and planted in the chicken run. With the rain both plants seemed to have survived the drastic treatment.

There's one old tree growing near a local beach - it had a girth wider than my arms outstretched and a good ten metres tall. The owner didn't like it growing so near his power lines so he took to it with a chain saw - about as high as he could reach. Within six months it had revived and was flowering so profusely. In these sorts of climatic conditions they are about as tough as trees come.

Good luck with yours, they'r healthy looking plants.