How to implement Auto Layout to fit a StackView with UIButtons inside UIView?

I create a custom Numpad keyboard through xib and stuck on its proper layout.

NumpadView has 4 rows in total: 3 rows with 5 buttons and 1 last row with 4 buttons.

Here is how it looks in a xib now:

I want to make all the UIButtons round with Aspect Ratio 1:1 and for NumpadView to be able to fit all buttons properly regardless of the height it can be initialized with.

I found a great answer with a similar problem on Stack Overflow: CLICK and followed it. On the upper screenshot you can see constraints I made based on its best answer.

The problem I have:

UIButtons don't change the size and fit the NumpadView frame (by height):

And If I will add a top constraint to a GridView then Aspect Ratio on UIButtons doesn't work:

I need the NumpadView to be able to fit all UIButtons inside and the buttons should stay round regardless of NumpadView height. How can I modify the Auto Layout to be able to solve this task?

Test project to play with: Github