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This tree is probably not getting enough light, so it has become spindly. Can you find a way to give it more light, such as placing it near a south-facing window or giving it an extra lamp?

Meanwhile, you can prune it back (cut off the top, leaving just a few leaves) to encourage more bushy growth instead of just one stem. Or, you can cut off the entire top section and root it to re-grow a shorter plant.

https://www.gardeningknowhow.com/garden-how-to/projects/rooting-plant-cuttings.htm

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/plumeria-pruning-info.htm

https://www.gardeningknowhow.com/houseplants/hpgen/houseplant-troubleshooting.htm

I am not sure why your plumeria didn't continue blooming during the summer. It sounds like you are following the correct steps to promote blooms. Some varieties may flower only in spring. By now, your plumeria would be done blooming in any case.

These articles provide some advice:

https://aggie-horticulture.tamu.edu/galveston/weekly_Q&A/f&b_22.htm
https://www.gardeningknowhow.com/ornamental/flowers/plumeria/plumeria-does-not-bloom.htm

Follow the advice in this article for pruning. Simply remove part of the two long branches to promote more bushy growth from the bottom.

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/plumeria-pruning-info.htm

I do think pruning would reduce flowering next year, but it would be worth it in the long run.

Is your plumeria receiving full sun? Plumeria enjoy as much sun as possible and need 6 hours of sun at least, preferably 10-12 hours to grow vigorously. Indoors, this can be supplemented with grow lights.

If it is receiving plenty of sun, and you are following the other guidelines for growing plumeria found in the articles below, I am not sure what the problem is. It sounds like yours is still capable of growing, so perhaps it just needs some time.

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria.htm
http://edis.ifas.ufl.edu/ep489

If the weather is beginning to turn cooler in your area, this could be the cause. Plumeria naturally goes dormant during winters, even in quite warm regions, and its leaves will gradually yellow and fall off.

Other possible causes include overwatering or poorly-draining soil in its pot, or a mild infestation of spider mites:
https://www.gardeningknowhow.com/plant-problems/pests/insects/spider-mite-treatment.htm

Plumeria have some specific watering and fertilizer needs during their active growth period.
These articles will help.

https://www.gardeningknowhow.com/ornamental/flowers/plumeria/growing-plumeria.htm
https://www.gardeningknowhow.com/ornamental/flowers/plumeria/how-to-fertilize-plumeria.htm
https://www.gardeningknowhow.com/ornamental/flowers/plumeria/plumeria-pruning-info.htm

Certain staking methods promote strong trunk development, while other methods can actually weaken the trunk. See the diagrams in these articles about proper staking and guying techniques. If the stake with your tree doesn't look like any of the diagrams (especially if the stake is placed right against the trunk, or if it is attached so that the trunk cannot naturally move with the wind) it is probably doing more harm than good.

If so, change your method to one of the proper staking/guying methods shown, and your tree should begin to strengthen. Then, remove the stake as soon as the tree can stand on its own.

http://www.myminnesotawoods.umn.edu/2008/12/staking-and-guying-trees-best-materials-and-technique/
https://www2.tntech.edu/tlcfortrees/mulching_staking.htm