ASP.Net Strongly Named App with external DLL Error of Strongly Assembly Required

I need to reproduce an old web site made in WebForm to ASP.Net MVC.

For now everything were going smoothly but in the last few pages, I've been required to add few DLL that were used by the old project.

I know that my ASP.Net app is signed with a strong name because I have my .snk and in the properties of the project, I see the snk selected in "Signing" dropdown.

The dll are inside my project in Libs/xxxxxx.dll and I added the reference in my project.

But when I try to use them, I get this error

Impossible de charger le fichier ou l'assembly 'XXXXXXX' ou une de ses dépendances. Un assembly avec un nom fort est requis. (Exception de HRESULT : 0x80131044)

My traduction to english:

Impossible to load the file or assembly "XXXXXXX" or one of his dependancies. An assembly with a strong name is required. (Exception HRESULT : 0x80131044)

I'm new to this "strong name assembly" things so i'm not sure what is required to make it use the DLL!

Anyone can help me with this problem!