I'm assuming that quote is in reference to:
If that is the case, then make sure you have followed the post installation instructions (and maybe go back and confirm you have followed all the steps prior to that). The very first link in that page shows you the general steps on how to set the Pihole as the DNS server for your DHCP hosts.
I should note that it is not a step-by-step guide for every make router out there, so I would heavily advise that if you're not comfortable with making changes and don't know how to make backups, that you should either get some help or hold off until you've done your research and are knowledgeable with the steps ahead.
It seems to me that you haven't set the Pihole as the DNS server inside the router - because of that no clients are being assigned this information via DHCP.