Watch blueprint communication tutorial.
All your problems are from fact that unreal uses object oriented programming, for most of things.
So you need reference to another object, then cast to its type then unreal knows what is inside referenced memory.
You somehow must tell UMG reference pointers to both static meshes.
Easiest way is if player controller finds those meshes and stores reference.
Then UMG can get owner player controller, and read references from there.
And watch blueprint communication.
Then dig into references and pointers, you should understand how that stuff works or you are into plenty of pain later.