Unity IoC : Create instance of an interface without constructor dependency injection

I am kind of new to Unity and DI terminology, hence trying to understand how does it work. I have following code which implements DI using Unity container.

public class DashboardService: IDashboardService
{
    private readonly IRepository<USERROLE> repoUserRole;
    private readonly IRepository<INSTITUTION> repoInstitution;

    public DashboardService(
        IRepository<USERROLE> repoUserRole, IRepository<INSTITUTION> repoInstitution)
    {
        this.repoUserRole = repoUserRole;
        this.repoInstitution = repoInstitution;
    }

    public List<USERROLE> GET(List<string> Id)
    {
        // Use repoUserRole object to get data from database
    }
}

This service is being called by the controller:

public class DashboardController : ApiController
{
    private readonly IDashboardService dashboardService;

    public DashboardController(IDashboardService dashboardService)
    {
        this.dashboardService = dashboardService;
        this.mapper = mapper;
    }

    //Action method which uses dashboardService object
}

Here is the Unity configuration:

var container = new UnityContainer();

container.RegisterType(typeof(IDashboardService), typeof(DashboardService))
.RegisterType(typeof(IRepository<>), typeof(Repository<>));

return container;

Questions:

1. As of now, my code works fine, but if I comment out the DashboardService constructor, I am getting the null repository objects.
2. I am resolving the dependencies in Unity for repository interfaces, so why I am getting null there?
3. Is there any way to pass the repository dependancy without using the constructor pattern?