Facebook Checkbox App in development mode requires advanced permissions for test messaging app admin - checkbox does not render at all

I am setting up a simple Facebook App following the Facebook Checkbox implementation guide (https://developers.facebook.com/docs/messenger-platform/discovery/checkbox-plugin/).

The goal obviously is to gather opt-ins from a simple HTML form. The problem is that even though I am logged in with the app admin account, the checkbox plugin fails to render with the following error message:

"Your app does not have the permission to message the user. Before approved for pages_messaging permission by App Review, you can only message people who have roles on the app."

I am sure that the frontend is correct, as I have tried swapping the appId and pageId from a working third party implementation and the error still occurs.

I cannot request an App Review, as I have no way of showcasing the app functionality without the checkbox rendering, nor is the development finalized.

The webhooks are set up correctly (tested with the dummy request). The app is added to a page, subscribed to opt-ins, and has standard pages_messaging permissions.

Can anyone see what might be missing?

Thanks, George