Reusable elements have a different “world” of its own. You won’t be able to reference to the elements inside the reusable element.
Here’s a great thread about reusable elements by @petter A guide to advanced use of reusables
You could add a state in your reusable element and reference it in your RG’s constraints.
You could have a workflow where if dropdown’s value is changed, set the state to dropdown’s value