I have already connected WhatsApp Business API with Stella, but how can I connect to a Facebook Page?
Customers can only add a new WhatsApp Business number (with the Business App) in Facebook Business, but not WhatsApp Business API (WABA).
Since you have already created WABA, please connect the WhatsApp Business API number to the Facebook page instead.
Your WhatsApp number will receive a confirmation code sent from Facebook and you could check the code in the conversation history in Stella.
For more details, please click here for the Facebook reference link.