Impossible to use RocketCDN API Key from a previous WP Rocket subscription

[WordPresseur]

For a WP Rocket customer who subscribed to RocketCDN via WP Rocket but don't use WP Rocket anymore, it won't be impossible to activate their RocketCDN subscription in the Standalone plugin, even with the API Key.

They get this error:
"Your website is not yet synchronized with your subscription. Please add your website from your RocketCDN account page."

To Reproduce
Steps to reproduce the behavior:

1. Create a RocketCDN subscription from WP Rocket
2. Get your CNAME and the API Key from your RocketCDN profile: https://rocketcdn.me/admin/customer/customer/
3. Uninstall WP Rocket and install RocketCDN plugin
4. Use the API Key in the plugin.
5. See error

Expected behavior
A RocketCDN API Key should work in the RocketCDN standalone plugin.

Additional context
HS ticket example: https://secure.helpscout.net/conversation/2518158413/476447/

Acceptance Criteria (for WP Media team use only)
Clear instructions for developers, to be added before the grooming

[CrochetFeve0251]

@WordPresseur I guess that issue would be more the saas rather than the plugin part. You might want to publish it here instead: https://github.com/wp-media/rocket-cdn