The usage scenario for the RT Box is to generate C code from a model and then compile this code for the special execution environment of the RT Box. A DLL that was compiled for your Windows computer (that uses a different processor and a different operating system) cannot be used on the RT Box.
The easiest solution is to either model the behavior of your DLL with PLECS blocks or to use a C-Script (which can be compiled for both your computer and the RT Box).