How to repair a concrete base?

Good morning,

I want to repair the fence base, which is made of concrete. Unfortunately, it has large gaps, and to make it straight, I'd have to add 5 centimeters of plaster in some places. The store recommended frost-resistant tile adhesive to simply repair the imperfections and apply the filler. That seems unusual to me, especially since it's not a flexible adhesive. But maybe you don't even need it outside?

How should I proceed?