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)

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