I'm also still experiencing some trouble getting Deco X20 to play together with my pihole, but at least it's functional now. Several things to keep in mind:
-
If you put the Deco in router mode, it will create its own subnet in which itself is both DHCP and DNS server - there does not seem to be any way to disable this. So even if you connect all your devices (including pihole) in the same subnet via Deco, there is no way to have your other devices automatically use that pihole DNS server.
-
If you put the Deco in access point mode but with the SmartDHCP option enabled, it still tends to run its own conflicting DHCP server.
-
The only way I found things to run reliably is put the Deco in access point mode and disable SmartDHCP. Also if you use a wired connection between the Deco's, it's recommended to connect the secondary Deco nodes via the main Deco (eg. don't connect them all directly to your router).
-
My remaining issue is that I cannot seem to control the IP address that pihole assigns to the Deco(s). I can see its MAC address in the static leases list (and it matches the incoming DHCPDISCOVER request), but still pihole assigns a random different IP address from the 'free-for-all' range. How could I diagnose this DHCP behavior?