I have custom controller like this:

using System.Web.Mvc;
using Umbraco.Web.Mvc;
using HttpGetAttribute = System.Web.Mvc.HttpGetAttribute;
using RouteAttribute = System.Web.Mvc.RouteAttribute;
using RoutePrefixAttribute = System.Web.Mvc.RoutePrefixAttribute;

namespace MyProject.Host.Controllers
{
    [RoutePrefix("partnerClient")]
    public class PartnerClientController : UmbracoController
    {
        [HttpGet]
        [Route("firstClient")]
        public ActionResult FirstClient()
        {
            return View(@"~/Views/Partner/FirstPartnerClientView.cshtml");
        }
    }
}

I am trying to call action {hostname}/partnerClient/firstClient
But I got an error:
Page not found
No umbraco document matches the URL '/partnerClient/firstClient'.

But when I use UmbracoApiController routing is ok

using Umbraco.Web.WebApi;
using HttpGetAttribute = System.Web.Http.HttpGetAttribute;
using RouteAttribute = System.Web.Http.RouteAttribute;
using RoutePrefixAttribute = System.Web.Http.RoutePrefixAttribute;

namespace MyProject.Host.Controllers
{
    [RoutePrefix("partnerClient")]
    public class PartnerClientController : UmbracoApiController
    {
        [HttpGet]
        [Route("firstClient")]
        public object FirstClient()
        {
            return "{test}";
        }
    }
}

I have to return View for this action and UmbracoApiController doesn't allow it.
How I should configure routing for UmbracoController? Or maybe I should use some other way to return some View?