The domain troubleshooting guide can help with most custom domain configuration issues. You might be able to use that guide to solve it before a human is available to help you. Then you can come back here and share the answer for bonus points.
You can also use v0 to narrow down the possibilities.
My colleagues and I would like to assist. Unfortunately, Threat Actors are abusing Vercel because they know that these URLs are left untouched for a long period of time. Please let’s connect via email and discuss.