Smart devices no longer function

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2

First, your IPv6 is not working:


*** [ DIAGNOSING ]: Name resolution (IPv6) using a random blocked domain and a known ad-serving domain
[✓] webbaoviet.com is fddc:d916:a734:db00:23b:9c3c:9cde:2ca0 via localhost (::1)
[✗] Failed to resolve webbaoviet.com via Pi-hole (fddc:d916:a734:db00:23b:9c3c:9cde:2ca0)
[✗] Failed to resolve doubleclick.com via a remote, public DNS server (2001:4860:4860::8888)

Next, you have some external modifications that are not Pi-hole:

*** [ DIAGNOSING ]: Setup variables
    SPEEDTEST_MODE=python
    SPEEDTESTSCHEDULE=4
    SPEEDTEST_SERVER=
    SPEEDTEST_CHART_DAYS=7

Next, you have set the lease time for DHCP clients to be infinite:

-rw-r--r-- 1 root root 488 Jul 24 13:14 /etc/dnsmasq.d/02-pihole-dhcp.conf
   dhcp-authoritative
   dhcp-range=192.168.1.180,192.168.1.200,infinite
   dhcp-option=option:router,192.168.1.1
   dhcp-leasefile=/etc/pihole/dhcp.leases
   domain=lan

Why are you shutting down the Pi and thus disabling the DHCP server? Once the Pi-hole is down you will have no way for clients to know hostnames.

Having a "secondary" DNS does not mean it's a fallback.