Yes you are right in your time zone they are still dormant.
The stem will not root only the bottom part. When you reported it did you notice any roots? If you have a picture that would help if you can post it here.
Depending on the length of the cutting I would only bury 2 to 3 inches. I keep the soil more to the dry side while still in dormancy to avoid root rot. Plumerias do not like wet feet. I also keep them in a warm sun when the weather is always above 50+. Then once 3 to 4 sets of leaves would open up I resume normal watering.
