Unfortunately my ISP-provided router doesn't let me change the DNS servers, I can just disable DHCP. I do see in the Android network settings that DNS 1 is my Pi-hole but DNS 2 is 8.8.4.4, just like the linked thread said.
After trying the solution in that thread (declaring a secondary DNS server in Pi-hole) and checking with the Network info app, it looks like the phone now has 2 DNS servers, both Pi-hole.
I'll test a little bit and update the post! Thank you!
BTW, one of those "Network Info" apps showed this and mentions local fe80:: IPv6. AFAIK I'm not using IPv6 anywhere and my router's only displays a IPv4.
(Also, what app do you recommend? there's a lot of them and I needed to download 3 to find one that wasn't trash)