Using the Pin feature, you can Pin a post between already published posts on Instagram to preview the feed.
However, there is no way to post new posts to the Instagram app retroactively.
• Add a new post with a photo to the feed
• Tap the Select button → Select a new post → Tap ‘Pin’.
• Set the post date and time in the past.
Pin feature is available to all users without restrictions and is suitable for all types of Instagram accounts.