You have to put the IP of a real DNS server (this is the same when you setup any machine). I'm not sure how that's difficult - feel free to ask specific questions though.
See:
- OpenWrt AdGuard Home 101 ( DNSMASQ )
- OpenWrt AdGuard Home 101 ( UNBOUND ) - or
- https://forum.openwrt.org/search?q=adguard
Hope this helps.