Can I reference non-WPF classes in a resource file?

Question

I noticed I can reference namespaces in another project (the view folder in mvvm) but the namespaces and classes in the sibling view model and model folders are inaccessible, even if I enter it manually, it claims it doesn't exist. I believe this is not standard behavior because those view models were accessible in the same file until I refactored the codebase and split each module into it's own project. I've added all 3 current projects in the projects reference. I found a reference to the assembly reference of the project itself clr-namespace:Solution.Project;assembly=Project so I tried using it thus DataType="{x:Type myns:TargetNs.TargetClass}" but it says nested types are not supported for this attribute, so I tried using it as a standalone resource i.e yet it says the name TargetNs does not exist in that namespace. How can I access those folders? What could the problem be? Many thanks.