Sorry, it's not entirely clear (at least to me) what specifically you want to change.
If you want your Almond+ to use a DNS server other than the one configured by your ISP, that's configurable in the normal web UI under Router -> Network -> "Use custom DNS Servers".
If you want to add custom DNS names visible to clients that use your Almond+ as a DNS server, you can do that in the OpenWRT web UI. Go to Advanced -> OpenWRT in the normal web UI, then log in again. Choose "Hostnames" from the "Network" menu. Click Add, enter a hostname and IP address, and click Save.