Help, please!

Hello everyone!
I'm back with more questions on my "only child". It's been staying outside for a couple of months now since the night time temp rarely dip below 50. Since I got it in December of last year, it only have a total of ONE leaf unfurl, and when that leaf fell off about a month ago or so, it seemed to have gone downhill from there. I got this as a rooted cuttings back then, but I never checked if it's true or not since it was sent in its pot, I assumed it has roots, and didn't want to disturb it.
I couldn't stand that it's not doing anything else, when there are new leaf tips, they seem to turn black and fall, so I unpotted it today, and this is what I found (see pic). No sign of roots whatsoever. If it did have roots before don't you think I'd find some roots remains in the soil? There was none, which I think is so odd. Up until 2 months ago, I used bottom heat inside the house, so it might had some roots since a leaf was able to unfurl, but maybe I took it out too early and the roots decomposed, that's why I didn't see any sign of it all all today? Now that the weather has gone up considerably during the day, I'm hoping that this can be rooted.
The bottom part looks black in the picture, but I didn't wash it thoroughly, so there are soil left there. It feels firm in that area though, not mushy at all. Some part of the stem feels a bit soft, but more like underwatered type soft/wrinkling instead of rot, but then again I'm new at this, so not sure.
I repotted it into a black plastic pot, and set it under full sun with plastic bag covering for extra humidity.
If it helps, this is suppose to be "Kona Rainbow" although I can't find any info/picture on it anywhere.

Any advice and suggestions will be greatly appreciated. Thank you.

Bailey,I could be wrong but I thought I spotted a few tiny hair roots on the right side of the bottom.I probably would have dipped it into rooting hormone and potted her back up right to the hottest place you could find maybe on asphalt or concrete pavement in full sun if you have any.A trick I've started to use ,to get cheap bottom heat.I take 12" X 12" patio blocks and find a place in full sun and then take one asphalt roofing shingle black and just put that on top of the patio block.Then set my pot on top of them and a bamboo stake(on the outside of the pot) to keep the pot from blowing over in a gust of wind.If you sink the patio block to ground level even better and let the lenght of the roofing shingle hang over the sides,you will warm up the ground on the sides and will hold more heat in. Don

Bailey,

Did you know that they grow/root best with night time temps in the 65-75* range?

Bailey, it could have had roots that rotted. If it wasn't watered well on the heat pad, they could have fried and dried up. If you could have taken a picture of the base close-up, I could tell you whether it was rooting or not by the way the cambium was or wasn't protruding, but now that it is in a pot, don't disturb it. A plastic bag over the cutting in full sun is not a good idea. It will shrivel up and desiccate very fast in that condition. Just put it in full sun on the hottest surface you have and water well. Mist as you water. Don't let the soil dry out. If it was pushing out leaves, then it wanted to root, but the roots weren't able to sustain the leaf growth at that point. Just give it as much bottom heat as possible, and don't pull it up to check on the roots anymore because plumerias have fragile roots which break easily. Good luck!

Thank you Don, Dale, and Clare for the tips.

So, Clare, just to make sure, although it's now literally in a "cutting" stage again, it still needs to be water regularly, correct? I'm sure the root loss was caused by underwatering in my part, I put it on the same schedule as the succulents, every 7-10 days depending on the weather. I guess it doesn't agree, oops.

Thanks again for the suggestions!

Bailey, when a plumeria cutting is sitting on a heat mat, it needs that heat to form roots, and the soil will dry out very fast on a heat mat, and the roots need moist soil so watering regularly while cuttings are on a heat mat is important for the formation of roots. Once roots exist, they need to be hydrated and develop in moist soil. If they don't get enough water, they will dry up. I water my cuttings whenever the soil looks dry, which is about every other day for me here in Southern California. They are in full sun and sitting on hot gravel. At this point, your cutting has to be a bit stressed and runs the risk of desiccation so lots of bottom heat, lots of water, and frequent misting should help to bring it back to health. You might even add a little Superthrive or Vitamin B-1 to the water when you water.

Thank you very much, Clare.
I'll pay more attention to the watering needs for this poor thing, and hopefully be back with good news.

Good luck and let us know. It is hard to tell in the picture, but it looks like there is a root there on the far left side. If that is a root, then it should recover quickly as that root grows out and new ones develop.