If you've worked your way through the steps detailed in our set up doc here, and your domain whitelabelling still isn't working, then it might be an issue with restrictions that are in place with your domain.
We handle our SSL certificates through Lets Encrypt, and a small number of our users find they have restrictions in place that block it. This prevents whitelabelling from working correctly.
If this is the problem, then you can add a CAA record to your domain.
It will look something like this:
[YOUR DOMAIN] IN CAA 0 issue "letsencrypt.org"
Adding this CAA record should solve your problem.
If there are no restrictions in place and whitelabelling still doesn't work, please contact us at firstname.lastname@example.org and we'll help you find a solution.