I'm having a similar setup (Main Router OpenWrt >> AP OpenWrt). I had to untick "Filter private" under Network >> DHCP and DNS >> Advanced Settings and put the DNS server and Gateway IP of my main Router into the LAN Interface to show the real hostnames on my AP. For some reason it doesn't work for my guest LAN/Wifi Interface/Network, so all the guest clients which are connected to my AP are showing as "?" in Luci. I guess it's related to my ebtable setup (to fully isolate guest clients)...
Afaik you have to untick "Filter private" in the Advanced Settings under the Network >> DHCP and DNS (Do not forward reverse lookups for local networks).
Please let us know if it works for you...