unfortunately, it often happens that Facebook bans an account immediately (or deny ads to be published) even if you meet the requirements and your products don't violate the policies. It's very strange (they aren't checking the ads nor the accounts manually so yes, therefore it happens that one of their bots bans the account).
Is there any note explaining why the ads have been rejected? Or is it the entire ad account?
There is no live chat which makes it even more difficult to chat/talk to someone. Did you use this link for submitting a review? https://www.facebook.com/help/contact/2026068680760273
At least, you can give it a try (for one of my clients it helped in the end). I would wait for a couple of days and if you get no answer then I would create a new Business Manager, Ad account and also use another credit card (sign up via another email) and then connect with your Shopify store. Sometimes this helps, sometimes not. You never know. (Then, you will need to remove the Instagram account from the “old” BM and add it to the newly created one).
It's sad that contacting Facebook support takes always so long and sometimes they don't even answer to you.
Give it a try and you will see.