To be able to connect and post to Instagram, you have to first make sure that the Instagram account you are trying to connect is a Professional Instagram Account and then you have to connect that Instagram account to its related Facebook page since all posting to Instagram is done through Facebook.
Connect Your Facebook Page and Instagram Professional Account
To connect your Facebook Page and Professional Instagram account you have to:
Go to Facebook Page and click Page Settings in the bottom left.
Click Instagram in the left column.
Click Connect Account.
If you’d like, toggle Allow access to Instagram messages in Inbox on or off. When on, it provides access to people who manage your Page to view and respond to Instagram messages but isn’t required for connecting/posting purposes.
Click Continue.
Log in with the correct Instagram Business account.
Posting an Image or Video to Instagram
After you've connected your Business/Professional Instagram account in the "Connections" tab, the only place where you can choose to post to Instagram is when you go to the "Custom Post" tab and then choose to either post a "Photo" or a "Video".
You won't see the option to post to Instagram when in the "Content" tab or as a "link" in the "Custom Post" tab because, unlike most other social media platforms, Instagram doesn't allow people to post URLs/links to content.
The way that most people handle URLs is by going directly to their Instagram accounts where they allow you to put in one link in your profile that can link out to a website. Then on posts, people bring attention to that link by mentioning in their posts to "Check out the link in our profile".
