I have just gotten a reply from my ISP:
Thank you for your e-mail. There has been no change to the routers policies.
It's correct, that the DNS on your PC will be 192.168.0.1, unless you change the DNS settings directly on the PC's NIC.
Your PC's DNS will be the router, and the routers DNS will be either default or whatever you choose to change it to.
Come to think of it, when I do set custom DNS settings on my router as 192.168.0.24 (the Orange Pi w/ the Pi-Hole), I do get ads blocked. So despite my ipconfig /all stating that my PC will use as a DNS my router at 192.168.0.1, the router still forwards the request to the Pi-Hole and gets its response from there.
So the question remains why I can't access pi.hole/admin and why I am cannot seem to get proper query logging as I used to just a few weeks ago. I used to be able to tell each and every device's traffic and get the correct separate counters.
I was leaning toward the fault being in a new update of the router, but given that did not happen, it's more likely a Pi-Hole or an OS issue. Just to put it out there, I have tried uninstalling pihole by running
pihole uninstall
and following it with
sudo rm -rf /etc/.pihole /etc/pihole /opt/pihole /usr/bin/pihole-FTL /usr/local/bin/pihole /var/www/html/pihole
as mentioned by How to fully uninstall pihole - #2 by RamSet
The only thing is that I DID NOT delete the associated packages it asks me to review one by one. I avoided deleting them because I am uncertain what each and every one of them is used for and I do not wish to mess up my headless server. However, given the circumstances, when I get home today, I will remove each and every one of them during the pihole uninstall and then also run the second command. Other than flashing a brand new OS on the server, I don't think I can get a cleaner install.