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.

https://vercel.com/docs/projects/domains/troubleshooting

vercel.com

Troubleshooting domains

Learn about common reasons for domain misconfigurations and how to troubleshoot your domain on Vercel.

You can also use v0 to narrow down the possibilities.

Cross-posting this as it may be helpful

https://vercel.com/kb/guide/how-to-resolve-ip-blocking-issues

vercel.com

How to resolve IP blocking issues | Vercel Knowledge Base

Learn to troubleshoot IP blocking issues for both shared and personal networks.