This is how I was able to add local DNS entries on my Asus RT-AC68R.
- Install AsusWrt-Merlin firmware: https://github.com/RMerl/asuswrt-merlin
- SSH into router
- "vi /jffs/configs/dnsmasq.conf.add"
- add line "address=/hostname.domain/192.168.1.X"
This is how I was able to add local DNS entries on my Asus RT-AC68R.