AWS Amplify DNS issue

This issue with your Amplify custom domain not being accessible on some networks could be related to several factors:

1. DNS Propagation Time: DNS changes can take up to 48 hours to propagate globally. Some users might be accessing DNS servers that haven't updated yet.

2. CloudFront Distribution Issues: When you set up a custom domain in Amplify, it creates a CloudFront distribution behind the scenes. There could be an issue with how your domain is associated with this distribution.

3. CNAME Configuration: Ensure both the validation CNAME record and the subdomain CNAME record are correctly entered in your Namecheap DNS settings without typos or extra spaces.

4. DNS Provider Settings: If you're using Namecheap's proxy services (similar to Cloudflare), try setting the proxy status to "DNS only" (not proxied) as this can interfere with AWS's domain validation.

5. Conflicting Records: Check if there are any conflicting DNS records for the same domain/subdomain pointing elsewhere.

Troubleshooting steps:

• Use tools like dig or online DNS lookup tools to verify your CNAME records are properly configured and visible from different locations
• Check if your domain is associated with any previous CloudFront distributions
• Try invalidating the cache in the Amplify console
• If the issue persists, you might need to delete the custom domain from Amplify and set it up again, ensuring all DNS records are correctly configured in Namecheap immediately after

If these steps don't resolve the issue, you may need to contact AWS Support for further investigation of your specific configuration.
Sources
Amplify custom domain setup issue | AWS re:Post
Timeout when creating custom domain on AWS amplify | AWS re:Post
CNAME problem deploying a custom domain in Amplify | AWS re:Post