I tried that, devices just request to alternate dns when they get nothing from pihole. I use adgh, and ig there is a setting where you can set the answer to blocked stuff, like 0.0.0.0, empty…etc. if you set thay to 0.0.0.0, devices won’t query 2nd dns (i hope) when adgh is up. But it is best to have a 2nd pihole/adgh, i have one on my proxmox and another on a pi, synced with adgh sync