Club 5028 website does not resolve for some users

It looks like the dns they are using is not resolving the domain.

The dns they are showing are internal network DNS possibly their routers.

With windows the tools are limited to find the DNS resolving path.

1) ask your user to reboot their modem and router. See if that helps

2) ask your user to contact their ISP and find out what the IPs of their DNS are.

3) have your users change their DNS IP to the google DNS IPs 8.8.8.8 and 8.8.4.4
developers.google.com/speed/public-dns/docs/using