Using the web app from your mobile device simplifies the process of
reposting media to Instagram. Here's how it works when you're on the
media's page after you pasted the link from Instagram.
Tap the Download option in the dialog
If the download dialog doesn't show, you may have already
downloaded the media. You can access downloads by tapping the icon
next to the address bar in Safari.
Tap the Share button in the bottom left corner
From the Share menu, you can
Tap the Instagram icon and post the media to your feed or
stories.
Save the media in your photos library.
Tap the Download button
Tap the Open option the download notification
Tap the Share button in the top right corner
From the Share menu, you can
Tap the Instagram icon and post the media to your feed or
stories.