The URL needs to be accessible from all CHILI servers. Make sure that your firewall settings allow a connection and test the URL from the servers themselves.A way to get dynamic assets (images) into CHILI publisher, is to call a (public) URL. Or, at least a URL that is reachable from the CHILI instance.
Steps to take
Define a DAP URL in the BackOffice
Create a variable in your document to add to your URL
Use an Image Frame in your document, containing the variable
DAP URL Definition
Go to Dynamic Asset Providers