Right about the same time you sent this, I discovered a setting in the router under 'Advanced' -> 'DHCP/DNS' (also shown in an image in the topic you linked" that is called "Use Internal DNS". I solved my issue by turning that off, though I did try that before I posted here. I unchecked that box, saved, then navigated to 'Basic' -> 'Network', verified that pihole was specified as the destination for DNS (made no changes), then simply saved that page again. Seemed to resolve it 
Anyway, however, thank you all for your help, Blockhead, jfb, and rdwebdesign.