If you are able to reach the endpoint via web browser, verify what is the proxy settings for the browser.
Open the browser settings and LAN settings.
If the option enabled is Use Automatic Configuration Script, copy the whole address specified and try to access it via browser.
A PAC file will be downloaded.
Try to open this file with a text editor.
Looking at the PAC file try to identify your proxy details.
Here is more information about proxy.pac file: Autoconfigure Scripts for Proxy Settings