G Suite: I cannot access any subdomains, e.g. mail.domainname.com

If you want to access G Suite email, calendar, docs at the respective subdomains, e.g. mail.yourdomain.com or calendar.google.com, you need to add this in your G Suite dashboard: 

https://www.google.com/a/cpanel/REPLACE_WITH_YOUR_DOMAIN.COM/CustomUrl