Cannot import native plugin in hololens app

I need to include in my hololens application (unity application for uwp with holotoolkit package) a native library written in c++. I followed this tutorial to make the library http://longqian.me/2017/02/24/eigen-in-unity/

But in the extern function call I get this error:

Exception thrown: 'System.DllNotFoundException' in Assembly-CSharp.dll
An exception of type 'System.DllNotFoundException' occurred in Assembly-CSharp.dll but was not handled in user code
Unable to load DLL 'Eigen': The specified module could not be found. (Exception from HRESULT: 0x8007007E)

The library is in Assets/Plugins/WSA/x86 and it is set in unity for x86 (I tried to set also sdk and scripting backend and the don't process option). I tried to use the kernel32 LoadLibrary with the name of the dll or the full path. When I build, the dll is copied into the bin/x86/Debug folder.

I tried to do the same with a regular UWP application for PC but there the same code works. How can I fix this?