Even when you sign up for Instagram using your Facebook account, the accounts are not automatically linked. You need to manually connect your Instagram account to your Facebook page.
Please refer to this Support Article on the Meta Business Help Center website ( applicable for web browser and mobile App user).
The following is using Web Browser:
1. Log in to your Facebook account using your browser
2. Click your Profile button (top right)
3. Choose Settings & Privacy
4. Click Settings
5. In the Permission section, click linked accounts
6. Click View
7. Click connect to your selected Instagram account
