Using an external library gives DllNotFoundException

Hi,

I'm trying to use https://github.com/entrepreneur-interet-general/H3.Standard with Unity. I've added the package with NuGet, but when I'm trying to use the methods, I get a DllNotFoundException about h3lib.dll. The InstanciateNativeLibrary() method given in the H3Standard puts the h3lib.dll to the project root, and not inside the Assets folder. If I add the .dll manually, and try to add the reference in Visual Studio, I get "File 'h3lib.dll' is not a valid .Net Assembly". Any tips on how I could get this working with Unity? If you can provide a sample project where this is working with all the steps to install, I'd be grateful! I've also raised the issue in Github, but as they might not be familiar with Unity, I thought it best to ask here too.