Using firstSelected not working with UI buttons

I am trying to make a quite simple menu, in which the button sprite changes when its selected, having two different sprites for when is and is not selected. The logic for the sprite changed is done and working properly, but I can’t manage to get the first button in the menu selected by default when you first open it, neither can I select it with the keyboard, having to use the mouse to click on one.

I have tried using EventSystem and choosing the button as firstSelected, but that didn’t help. I’ve also tried to manually script the button as selected, but it didn’t work either.