Hi Amber,
Since, you're able to access the metadata of the service through the browser, it seems VPN is not the issue here.
The best way to troubleshoot the issue is to use a browser based REST client tool like Postman (for Chrome) for issuing a POST call to the service. The client shall ask you for an authorization to access the front end server where your oDATA service is hosted. You may pass the Username and Password in the Service URL (preferably a separate service user ID for security).
Once you're able to make a POST call, you shall be sure that the service is working fine and accessible externally. After this, you may integrate it with your UI code.
Regards,
Shalabh