I have your "Tips for Proper Mandevilla Care" but don't know what brand fertilizer to buy.
Please Give Me a Name of a High Phosphorous, Water Soluble Fertilizer
Certified GKH Gardening Expert
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