Opening, sharing, and managing OneDrive-stored documents directly within Microsoft Teams
Dear @Jack Triassi CPA,
Thank you for posting your question in the Microsoft Q&A forum.
Based on your description, you cannot open and see documents from external source and receive warning message.
Could you please share with me what specific warning message is?
Have you tried to check to your IT/admin to ask about policies?
Please prefer to this article to understand more about this issue Representation of users outside your organization
Looking forward to your update.