It is probably not related to openwrt but maybe so, on my pixel7, I try to add a customs dns like cloudlfare ou quad9, but only google dns works, if I add cloudflare, quad9, no wifi connection EXCEPT on my guest network only, someone has already saw that problem?
This is indeed off topic for the OpenWrt forums if you're talking about configuration of the Pixel or other devices.
You can advertise a preferred DNS server using DHCP option 6. Or, if you want to enforce this at a router level, you can setup dns hijacking (this works for standard DNS over port 53; it's a much more complex problem to solve for DoH/DoT):
yep but as I said, only guest network works when I add customs dns servers in my phone. On my router I use dnsproxy.