If you use DNSChecker, you could find that domain
onlinexxx.com has been resolved to another public IP address instead of your Azure VM public IP (20.185..). This is the problem.
To access your websites for external users, you only need to purchase a public domain from a domain provider like GoDaddy or by an app service domain. then you can add
CNAME records to map your VM public IP address or FQDN in your DNS provider. You even don’t need to verify this domain in Azure AD.
In addition, you have the option to host your domain to the Azure DNS and delegate your domain in Azure, but this is not very necessary if you want to use your domain provider to host the DNS zones.
Please let me know if you need further help or you can get more DNS troubleshooting steps here.
CLICK HERE to find out more related problems solutions.