I do not see any DDNS button. I also do not see any documentation from Maraki stating that the MX67W does not have DDNS.
Should it be on the Appliance Status - Summary page?
I think the page the OP linked to needs an update as the configuration of DynDNS has moved. Nvm, the instructions are actually correct. It's just the "Configure DynDNS button" that's a bit unclear. It's in fact the little pencil.
It is. If you click on the pencil, you'll have the option of customizing the first part of the url and to disable dyndns if needed.