Probuilder not working properly,Probuilder selects full cube but not vertices and faces

Whenever I want to select face and vertice of a cube I cannot it is by default selecting the whole cube. And the face and vertice select tool is not working I think so pls help asap. I have the latest version of probuilder and pls suggest me the most stable version of unity and pro builder pls ,Whenever I have to select and delete faces in cube it always selects the whole cube the vertice and face selection buttons are not working properly pls help

I have the same issue, my objects have been done in AutoCAD first, then imported in 3d studio to assign a default material and then exported as .fbx file to unity. I need to use ProBuilder to assign different materials to different faces of the same object.
Anyone who knows how to fix it?

Bests
Margherita