If you update the plugin to the latest version (Pro: 2.2), and then obtain a new Access Token by following these directions (https://smashballoon.com/updating-your-instagram-access-token/), then it should solve the problem for you.
If you have any trouble updating the plugin then please follow the directions here (https://smashballoon.com/updating-the-instagram-feed-pro-wordpress-plugin/)
Let me know if you still have any trouble.
Thanks,