DllNotFoundException, C++ dll used with an assembly, not possible?

Hi, I'm kind of new to things like handeling dll in unity and stuff. Here's my problem I need Unity and an over application to communicate with eachother. To do so I have an API made of aC++ dll from that other application, a lib and an header. There called API_C.1.6.dll, API_C.lib and API_DLL_C.h, I also have a second dll called API_NET.dll that is an assembly that does all the DLLIMPORTS for me so that I just have to add a reference to that dll in VS2013 and I can use all the fonction from the first dll.

Here is what the API_DLL_C.h looks like:

 #ifndef __API_CBINDING_h__
 #define __API_CBINDING_h__
 
 //! use this compilation option to create API.dll ///
 #if defined(API_STATIC) || defined(__linux__)
 #define declAPI
 #else
 #ifdef API_EXPORTS
 #define declAPI __declspec(dllexport)
 #else 
 #define declAPI __declspec(dllimport)
 #endif
 #endif
 
 #include API_DLL_Enums.h"
 
 #define MAX_DATA_ID_STRING_SIZE 256
 
 #ifndef API_STATIC
 #ifdef __cplusplus 
 extern "C" {
 # endif
 #endif
 
     declAPI int Process_InitParams(const char* ProcName, const char* ConfigName, float Frequency);
 
 ...(other fonctions)
 }

Here is my cs files that uses the dll:

 using UnityEngine;
 using System;
 using System.IO;
 using API_NET;//from the assembly
 
 
 public class TestComAPIUnity: MonoBehaviour {
 
     // Use this for initialization
     void Start()
     {
         plugin_API.Process_InitParams("TESTUNITY", "DEFAULT", 30f);
         
     }
 
     // Update is called once per frame
     void Update () {
        
     }
 }

This is the simpliest communication between Unity and my other application that can be done with the API.

With this there is no compilation errors but when I try to run in the editor I have this error:

DllNotFoundException: API_C.1.6.dll API_NET.plugin_APIPINVOKE+SWIGExceptionHelper..cctor () Rethrow as TypeInitializationException: An exception was thrown by the type initializer for SWIGExceptionHelper API_NET.plugin_APIPINVOKE..cctor () Rethrow as TypeInitializationException: An exception was thrown by the type initializer for API_NET.plugin_APIPINVOKE API_NET.plugin_API..cctor () Rethrow as TypeInitializationException: An exception was thrown by the type initializer for API_NET.plugin_API TestComAPIUnity.Start () (at Assets/TestComAPIUnity.cs:12)

What I understand(I think) is that when I call the function, the assembly does it's job and try to find API_C.1.6.dll but can't find it. Just to be sure I build the project the dll is in the project_data\Plugins folder but when I use the exe with my application nothing happens. At some point I did do the DLLIMPORT myself and I had this kind of error:

Failed to load 'Assets/Plugins/x86_64/audiopluginvrunity.dll' with error 'The specified module could not be found. ', TestComAPIUnity.Start () (at Assets/TestComAPIUnity.cs:12) DllNotFoundException: API_C.1.6.dll'

I arrived at the conclusion that doing the DLLIMPORTS myself or not just changed where the error would appear in my script or in the assembly. I also checked if the dlls were 64bits like my OS and my UnityEditor, they are. I placed the API_C.1.6.dll in Assets, in Assets\Plugins, in Assets\Plugins\x86_x64(like the parameter in the plugin inspector), i placed it next to my exe and the assets folder, next to the API_NET.dll(in my application folders). Some people on the internet seemed to had problems because they didn't had Microsoft Visual C++ 2015 Redistributable, I have it and I have a licenced VS2013. I also tried to change my environment variable PATH at runtime with a static constructor but it changed nothing.

I'm starting to run out of ideas, so I ask you guys for help. By the way, judging by the number of post and articles about it, a lot of people seem to have trouble with handeling DLLs in Unity. Thanks

Vincent P.S: because of confidentiality policy I had to change names and stuff so there may be one or to mistakes, sorry and forgive my not that good english