Only that first Pi-hole of yours will be aware of client hostnames associated with DHCP leases it has issued, provided a client did present a hostname during DHCP lease negotiation (or it has learned a name by some other means).
Querying your .252 Pi-hole will likely result in failures.
Let's verify that by running a reverse lookup through each of your Pi-holes:
Understood. It is my impression that the AT&T router is the primary cause of the issues and is assigning the names itself, but cannot resolve them as it is not the DHCP server. The second Pi-Hole was put in place simply to provide a secondary DNS to restrict the router from defaulting back to something else in case of a primary Pi-Hole failure.
I've been continuing to mess with it and have (at least temporarily) gotten it working by creating /etc/localdns.list, pointing to it in /etc/dnsmasq.d/99-extra-dns.conf with addn-hosts=/etc/localdns.list, and creating a new pihole-FTL.db.
I've created these two files on both Pi-Holes so they can resolve as long as network devices maintain their IP (not the best practice, but works without me breaking anything else).
I will be keeping an eye on this to see if a power cycle on either/both Pi-Holes has an impact, as well as to see if new devices on the network are able to resolve their hostnames or not. A concern I had had was that the DHCP leases from the router were not properly expiring when attempting to have devices connect through the Pi-Hole's DHCP server. Powering them off for a time in excess of the router's DHCP lease duration seems to be allowing some hostname resolution, but that has not been 100% successful. Further, setting static DHCP leases via Pi-Hole seemed to break the hostname resolution in the first few attempts, but I will need to try again now that I seemingly have gotten it working.
Let's verify that by running a reverse lookup through each of your Pi-holes:
This is actually what had me break out a Pi0W and set up the secondary Pi-Hole a few days back. .253 returned the proper hostname, while .254 returned the unknownXXXXXXX.attlocal.net. With both Pi-Holes set up now, both .253 and .252 return the hostname defined in /etc/localdns.list. Again, I will need to keep an eye on new/temporary devices on the network because I do not plan to add every hostname to localdns.list.
