There are countless ways to connect your Instagram account to your Facebook assets.
However, in order for ads made on your Facebook Page to be automatically connected to your instagram account, Facebook requires you to connect your Instagram directly from your Facebook Page.
To add an existing Instagram account to your Facebook Page:
Go to your Facebook Page.
Select Settings at the top of your Page.
To add an existing Instagram account to your Page, Select Log in.
Enter your Instagram account's username and password, and select Log in.
(sourced from: https://www.facebook.com/business/help/898752960195806)
Once done, please let your account manager know and they will re-sync your account :).