Did you set a break point in your add function and make sure the Add is called and the struct valid at that point?
It looks like it should work. But, I googled. The problem is 2D arrays are not supported in UE4 so the workarounds are a little janky.
It looks like you may need to set the inner array and can’t modify it in the outer array. So you may need to make a copy of the inner array, add your new element to it, then set that as the inner array. That is if I understood this AnswerHub post correctly.
https://answers.unrealengine.com/questions/138067/multidimensional-array-of-classes.html