Loss of appetite. Consulted few doctors!

I am having continuous loss of appetite and weight loss. For many years I was a skinny person I don't know why I never gained weight even I have tried many syrups and consulted few doctors regarding that but every one gave me some different different types of syrups. Believe me none if them worked. Around 1 year ago magically I started feeling more hunger and that I am having good appetite and started feeling hungry a lot. Increased the intake quantity of Dairy Products and gain my weight from 49 to 64 kgs but since October 2019 when I had a Dengue fever all my appetite gone and since then my weight is gradually decreasing and now I am back to 51-52 kgs. Even it's been months and again I tried syrups and every technique of taking small small meals whole day, nothing is working. For the record my height is 6 feet. I don't understand what I should take to what I should prefer to get my appetite back and gain weight. No syrups or protein intakes are working out. Suggestions please