On the same network, can you attach a device and navigate to the site using a local browser?... Assuming that works, likely you need proxy authentication to allow HTTP traffic to your signage panel. Check out the panel's proxy settings... I'll often add my own credentials just to test sites like this. Long term, you'd likely want to have an account created that allows HTTP traffic through your firewall for your fleet of devices.
As an alternative to deploying anything, test the web app using the browser source instead. The browser source uses the same web engine as the MagicInfo-Lite/S source. It makes local testing faster instead of taking a deployment path. Hope this helps!