NextDNS as i understand is a paid service and allows you to filter using their DNS services on both mobile (thus while out and about) and at home. Current recommendations are to use the AdGuard Home client on your router as a downstream NextDNS client as their client has issues. see here for more info : [How-To-Updated 2021] Installing AdGuardHome on OpenWrt - #6 by mercygroundabyss
The old way to do encrypted DNS was with stubby and unbound. AGH replaces both of them and does encrypted DNS internally so you do not need them anymore. Some people still prefer using them however it complicates your setup and duplicates the effort required. That being said, using adblock with unbound/stubby is preferable on smaller less powerful routers where space and cpu power is an issue.
AGH i tested with a BT Hub5 https://openwrt.org/toh/bt/homehub_v5a which has 128mb of ram. As long as you do not go wild with the amount of filters you add to it, then it is fine.
The wiki has more info on installation of the 107 version using opkg. There is also linked threads to manual processes like my updated thread on the forum here : [How-To-Updated 2021] Installing AdGuardHome on OpenWrt This thread uses the "edge" clients from AGH as they are updated more often.