PiHole and DoH (cloudflared) not working?

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Thanks for response.

All done directly from the Pihole CLI:
Well, there is no "eth0" listed, if I do a ip address show. There is only an adapter named "enxb827ebc729ca". This one has the IP address assigned. Is there any difference between the adapter name shown via CLI or Pihole Webfrontend? How do I change the IP of eth0?
If I do an ip -4 route show default | awk '{print $3}', I got 10.11.12.1 (which is my default router). From there it is pingable.

I did the test from my MacBook (connected via LAN only; WiFi turned off; only DNS was the Pihole IP) and from my iPhone (connected via WiFi; again only DNS was the Pihole).