Build an android apk from Unity3d project working with C/C++ dll

I made a Unity3d project working with C/C++ Dll. So first I made the dll in Visual studio.

 // mydll.cpp
 #define EXPORT_API __declspec(dllexport)
 extern "C" {
     int EXPORT_API add(int a, int b)
     {
         return a+b;
     }
 }

And I added this dll into my Unity3D project and called

 // C# script in Unity3D
 public class TestDll : MonoBehaviour 
 {
     [DllImport("mydll")]
     private static extern int add (int a, int b);
 
     void Start() {
         int a = add (100, 200);
     }
 }

This code works well and I got the correct result in Unity3D editor.

So I built this project to android platform and got an apk. I installed and run this apk on my android device but the app doesn't work. The app didn't run the Dll function.

Whats the matter? I tried to find out the solution in Google and here, but didn't get anything, yet. Please help me. Thanks.