Since I formatted the computer and changed internet provider, the pi-hole stopped working, that is, it stopped blocking the lists of undesirable sites.
Actual Behaviour:
PiHole does not respond to DNS queries
You don't seem to be using an official Pi-hole release?
Your debug log shows your installation to differ from Pi-hole:
*** [ DIAGNOSING ]: Web version
[✓] Version: v5.17
[i] Remotes: origin https://github.com/pi-hole/AdminLTE.git (fetch)
origin https://github.com/pi-hole/AdminLTE.git (push)
[i] Branch: master
[i] Commit: v5.17-0-gda2764e-dirty
[i] Status: M settings.php
[i] Diff: diff --git a/settings.php b/settings.php
index 7ff980f..3da6cf4 100644
--- a/settings.php
+++ b/settings.php
What's the output of the following commands:
nslookup pi.hole
nslookup flurry.com 192.168.15.9
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Yes, it is a modified version of pi-hole for windows.
obs: I'm very new to programming, I hope I've done what you asked correctly. And this modified version worked before formatting the computer and changing internet provider.
Running a few commands is not programming at all. 
You ran them correctly.
The second command sent a request to resolve the domain flurry.com (which we know to be blocked by Pi-hole's default list) explictily to your Pi-hole host's IPv4 address 192.168.15.9.
It came back with 0.0.0.0 as expected, demonstrating that Pi-hole has received and blocked it.
The output of your first command just requested resolution of pi.hole (a domain only known to your Pi-hole) through the default DNS resolver as used by your client.
It shows that your Windows client is not using Pi-hole for DNS, but rather an IPv6 address.
Consequently, it failed to resolve pi.hole (Non-existent domain (NXDOMAIN)).
In all likelihood, that IPv6 address is your router.
If your router is indeed advertising its own IPv6 address as DNS server, that would allow your clients to by-pass Pi-hole.
You'd have to find a way to configure your router to advertise your Pi-hole host machine's IPv6 as DNS server or to stop advertising its own.
You'd have to consult your router's documentation sources on further details for its IPv6 configuration options.
If your router doesn't support configuring IPv6 DNS, you could consider disabling IPv6 altogether.
If your router doesn't support that either, your clients will always be able to bypass Pi-hole via IPv6.
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Unfortunately my modem is all locked, I was all day yesterday trying to unlock but it seems there was an update in the firmware which made it impossible to make any changes; soon I will request a modem exchange.
@Bucking_Horn, thanks for the time invested in helping me.
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