Sharing Excel sheet via embed into public web page without login

I am having difficulty sharing an Excel file to a public web page using the embed iFrame function. It embeds fine for me and I can view the web page, however a public user (or me using an incognito browser) is getting prompted to login to office 365. I need to display the data without login. I have 

• Checked that sharing policies in Sharepoint admin centre are set to most permissive (both sharepoint and onedrive are set to "anyone" "Users can share files and folders using links that don't require sign-in."
• The Excel file is set (under manage access/share) "to anyone with link can view"

What am I missing?

Thanks for your help