Sounds like you might be doing all you can based on your location. Maybe add a heat mat under the container to keep the soil warm. Light watering with rainwater (you should have enough of that) and light or diluted liquid fertilizer may also be a good idea.

The inflos are looking great...Thank you Hopefully the soil is warm enough being inside. Still too cold here to move outside it will stay where it is. It get natural sunlight during the day and red/blue led grow light at night. Hopefully not too much light?

A heat mat would be one of your best investment in growing Plumeria's, especially that far north. People make the mistake of thinking that all they need is sunlight to grow but heat from below is crucial to get good growth and blooms.