file.Exist returns true on localhost and returns false on server c#

I’ve a grid that contains file list (that on another server) and download buttons for them on each row. When button clicked, file should be downloaded. When I click it on localhost, file.Exist returns true and I can download the file. However, when I try same button on server (IIS), file.Exist returns false and the file cannot be downloaded. (This server also can reach the file.) My code snippet is below;