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It is likely suffering from shock, which is a normal response in new plantings as they adjust. This article will help with that: https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm

It sounds like your plant has a fungus. Treat the plant with fungicide. I like to use neem oil as it is safe for people and pets and also will address any pest issues the plant may have. Here is more information on it: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

While there are numerous to choose from, you might try Miracle-Gro Water Soluble Bloom Booster Flower Food (15-30-15) or Shultz Bloom Plus Water Soluble Plant Food (10-54-10), though really any type will work as long as the phosphorus ratio is higher. You may also want to consider adding bone meal to the soil to increase phosphorus. Here is more information: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/bone-meal-fertilizer.htm

This is powdery mildew. This article will help:
https://www.gardeningknowhow.com/plant-problems/disease/get-the-cure-for-powdery-mildew.htm

There are many reasons this can happen. Yellow leaves are a sign of a stress of some kind. This article will explain more:
https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

I would check the pot. Make sure the water gets all the way into the soil in the container and that it is still draining properly.

There are a number of reasons why the leaves may turn yellow. One might be overwatering. Too much water causes yellowing of the leaves and mandevilla do not like overly wet feet. Another possibility could be too little water. If this is the case, the yellow leaves would be curling up as well. Mandevilla favor humid conditions. If your plant is exposed to the sun, the soil may be too hot. Try misting/spraying the leaves with water in the mornings and keep the soil moist (NOT wet). Mealy bugs can occasionally affect these plants, causing them to become weak and leading to yellow foliage. Finally, plant stress due to a change in location, temp., etc. can lead to yellow leaves.

Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm
https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm