The following thread is the current updated version to follow.
You should not need to change openwrt settings as those scripts do the work to move dnsmasq from port 53 (DNS port) to a backup port (54). This allows AGH to become your primary DNS server for your downstream LAN clients. Dnsmasq becomes your local dns server for your downstream clients. In short? public internet addresses are looked up via AGH, but local lan addresses eg laptop.lan are looked up via dnsmasq.
As for DNS settings, there are multiple places and reasons for this.
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dns settings for AGH should be the upstream servers you want to pull dns from. (AGH upstreams) - https://github.com/AdguardTeam/AdGuardHome/wiki/Configuration#upstreams
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Dns settings for OpenWrt is what the router uses to look up data. (WAN Dns)
This doesn't need to change but there is a script to change from your ISP dns for the router to a public one like Cloudflare or any other you choose. -
Dns settings for DHCP is what your downstream clients will use. (Lan DNS) This is passed by DHCP settings and is also set via those scripts.
That hopefully will help with your confusion.
:edit: tided the dns bit to make it more readable.