The leaves on my mandevilla plants are turning yellow and dropping off.
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). You may also want to consider moving your plant (if possible) to an area that doesn't receive direct sunlight for extended periods of time.
Here is an article that you may find helpful: https://www.gardeningknowhow.com/ornamental/vines/mandevilla/mandevilla-plant.htm