PiHole randomly working

[deHakkelaar]

@merlino

Then, the PiHole unbound guide tells me to run the following commands:
sudo systemctl disable --now unbound-resolvconf.service; sudo sed -Ei 's/^unbound_conf=/#unbound_conf=/' /etc/resolvconf.conf; sudo rm /etc/unbound/unbound.conf.d/resolvconf_resolvers.conf; sudo systemctl restart unbound however the second and third (can't remove a file that doesn't exist) command both fail.

The guide needs a little update.
Above is expected if below package isn't installed:

$ apt policy openresolv
openresolv:
  Installed: (none)
  Candidate: 3.12.0-3
  Version table:
     3.12.0-3 500
        500 http://deb.debian.org/debian bookworm/main amd64 Packages

$ apt-file list openresolv
[..]
openresolv: /etc/resolvconf.conf

@merlino

and, when testing, I've set the PiHole as the Secondary DNS Server too, but nothing changed

Have a read below:

Why should Pi-hole be my only DNS server?

Reduced Ad Blocking Capability The main reason you should Pi-hole as your only DNS server is that you will see increased performance in the blocking of ads. If you have two DNS servers (Pi-hole and something else), your network clients may not always query Pi-hole for name resolution. If a query happens to be answered from a non-Pi-hole DNS server, your block lists will not apply (since that DNS server doesn't know about them). All Queries Need To Go Through Pi-hole First Since other DNS se…

@merlino

This is the page on my modem related to DHCP/DNS Settings (after a modem reset):

Above you only have to enter the Pi-hole address in the "Primary DNS" field.
And if the router allows, you could also enter the same Pi-hole IP in the "Secondary DNS" field which is the default for the upcoming v6 release if Pi-hole's own DHCP service is enabled:

$ sudo pihole-FTL dhcp-discover
Scanning all your interfaces for DHCP servers
[..]
   dns-server: 10.0.0.2
   dns-server: 10.0.0.2

Usually no other router settings needs changing so leave them on default for diagnosing.

@merlino

I'm using a ZTE H6645P V2 modem

I used to have an ISP provided ZTE modem.
I believe it also supported IPv6.
Have you checked that bit?

You could try out a new Pi-hole v6 feature that scans for IPv6 RA advertised DNS servers.
For that you could down a preliminary v6 pihole-FTL binary with below (it wont get installed):

wget https://ftl.pi-hole.net/new/dhcpv6/pihole-FTL-arm64

Make it executeable:

chmod +x pihole-FTL-arm64

And let it rip:

sudo ./pihole-FTL-arm64 dhcp-discover

The bit you have to look for is "Recursive DNS server"