@akapal 

Hi, I used to have the same question and the Answer here helped a lot:

https://www.snbforums.com/threads/wan-dns-and-lan-dns.29940/

It might be a little old but it works for Deco as well:

Deco has itself run as a DNS proxy and the DHCP server page on the Deco APP will usually contain Deco LAN IP (that's the default when it's left empty but you could also customize your own server if needed), which means your clients will get this IP as the DNS server, and then Deco performs all DNS lookups using the DNS obtained either from the ISP or manually entered on the IPV4 page. Having this means that Deco can easily act as a caching server for DNS queries for the whole LAN, which can improve performances.

So the client will connect with cloudflares dns first, then to Google dns.

Best regards.