You can find a copy of the user manual for this router here:
I'll give it a look and see if I can find where you can change the DNS settings.
[EDIT] Well, after reviewing the document at this link and any others that I can find, I found that the user guide to be basically completely useless. It doesn't seem to offer any details on the menu options in the administration interface for this device. Without this information, I'm afraid I won't be able to assist you with where to look to make the DNS changes.
[EDIT, Part Deux] Looks like that this is one of those routers that does NOT allow you to change the DNS settings. In that case, Pi-hole recommends that you enable DHCP service on the Pi-hole to allow you to change the DNS settings to your network clients to use it. I'm not aware of any other workaround, other than getting a different router. Most other routers do not have this issue.