How do I stop cheesecake being runny without gelatine?

Short answer: full-fat cream cheese, everything cold, don’t overwhip, and give it a proper set.

A no-bake cheesecake sets on the strength of whipped cream and cream cheese, so the ingredients have to do the work. Usefull-fat cream cheese (low-fat won’t firm up), keep everything cold, and whip the cream and cheese only until it holds soft peaks — overwhipping splits it and it never sets right. Then be patient: six hours minimum, ideally overnight in the fridge. If you’re topping with something wet, like blended mango, drain it really well so it doesn’t slacken the surface.

Related:no-bake mango cheesecake · glossary:folding.