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A Soursop will drop it's leaves in temperature below 45 degrees F.

https://hort.purdue.edu/newcrop/morton/soursop.html

With soursops, even with good pollination, it is normal for many flowers to fall off and only a few to produce fruit, but the fruit production improves as the tree gets older. Hand pollination is possible but can be difficult. This article explains how to hand pollinate the related tree Atemoya in the Flowering Behavior section:

http://edis.ifas.ufl.edu/mg332

Soursop should be the same, but the schedule is: In the early evening, collect pollen from male flowers. Store it in the refrigerator overnight. Apply the pollen to the stigmas of the female flowers early the next morning.

Depending on it's age it just may not be old enough yet to produce fruit. This article should help
https://www.gardeningknowhow.com/edible/fruits/soursop/soursop-tree-care.htm

I fear it may be too cold for the tree to grow outside. Your best option might be to grow this in container. Here is an article That will give you more information on their care: https://www.gardeningknowhow.com/edible/fruits/soursop/soursop-tree-care.htm

This article will help you with care including fertilizer recommendations.

https://www.gardeningknowhow.com/edible/fruits/soursop/soursop-tree-care.htm

It won't be necessary to know exact NPK values, as they will be very similar. Finding fertilizers with exact values for each will get quite expensive, so a general fruit tree fertilizer will be sufficient once or twice per year. This can be found at many online retailers, or locally.