How do I clear my DNS cache when cmd: ipconfig/flushdns does not work to clean it?

There should be a space between ipconfig and /flushdns. Your message subject suggests you did not have one?

Or you could edit the registry as suggested in

http://www.updatexp.com/dns-windows-xp.html

Start> Run> type: services.msc then OK.
Make sure your DNS Client is enabled (either manual or automatic) and started.

depend on your system guide for 

https://geeksadvice.com/flush-dns/