Entry Point Not Found Exception

I'm trying to use a C++ unmanaged dll in a C# project and I'm getting an error when trying to call a function that says that entry point cannot be found.

public class Program
{

    static void Main(string[] args)
    {
        IntPtr testIntPtr = aaeonAPIOpen(0);            
        Console.WriteLine(testIntPtr.ToString());
    }

    [DllImport("aonAPI.dll")]
    public static extern unsafe IntPtr aaeonAPIOpen(uint reserved);
}

Here is the dumpbin for the function:

5    4 00001020 ?aaeonAPIOpen@@YAPAXK@Z

I changed the dll import to [DllImport("aonAPI.dll", EntryPoint="?aaeonAPIOpen")] and [DllImport("aonAPI.dll", EntryPoint="_aaeonAPIOpen")] and no luck.