After you've received the notification that it's time to publish a post, follow the instructions, long press to paste the captions and make sure you're publishing to the right Instagram Profile. If you're looking to select another profile, please tap on the right top bar and select the right account, see the GIF below.
Connect 2nd Instagram Profile
If you need to add more than one Instagram profile, please perform the following steps (available only from desktop):
Go to the Instagram.com
Log out of your current account
Log in another account you wish to connect
Go back to the Planable app (https://app.planable.io)
Click "Add Profile" and connect the second account