1. Go to this link and download FastSave for Instagram.
2. After download, open Fastsave for Instagram and click on open Instagram.Then Instagram will be open.
3. Now click on the three vertical dotted bar at the top right corner of the post that you want to download.
4. Now click on copy link.
5. Your Photo or video will be downloaded in the background.You can see that progress in the notification panel.
6.You can see the downloaded photos and videos in your gallery and as well as in my downloads folder at the below of open Instagram option in the FastSave application.