Hello
Thank you for your question and reaching out.
Perform a nslookup test on the Sharepoint URL from the OOS server to see if the URL path is incorrect. The full path to the document being viewed must be parsed/viewed by OOS, SharePoint, and the client. Repair the DNS if there is an issue.
If there is an error previewing the file from the external network due to your configuration using the internal HTTP method, ensure that the appropriate public SSL wildcard certificate is installed and TLS is enabled on the Office Online Server.
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