How to set up and use NextDNS with AdGuard app in Android

 You will no longer need the NextDNS app.

AdGuard is a powerful app which can monitor and block the traffic from your phone. I highly recommend it.
Settings - DNS Filtering - Select DNS server - Add custom DNS server.
Use what name you want in "DNS server name".
In "DNS upstream" you put any option you want between the Https, Tls or DNSCrypt. You need to use the prefix https:// or tls:// or sdns:// accordingly.
If you want to identify your device add the desired name as explained in setup tab (at the bottom) in your NextDNS account. DNSCrypt doesn't have this option (as far as I know).

Save and select.
You can add multiple profiles.

Some tinfoil advices. :)

A privacy oriented browser (using privacy add-ons like Tor browser and Firefox or Brave) use together with AdGuard app (which can be used with AdGuard VPN) and NextDNS can keep you safer (I don't know if you can even be safe) from Fingerprinting and tracking which I think are a real problem this days much more than adds which are only annoying. You can find literature about this subjects. Many thinks that if they use a VPN are anonymous. Read about browser fingerprint, device fingerprint, Bluetooth and Wi-Fi beacons, probes (Disable Bluetooth and Wi-Fi when you don't use them), the danger of using GAFAM (Google, Amazon, Facebook, Apple, Microsoft).  
Avoid unnecessary apps if you can use web site. Some apps can see your mobile network, Wi-Fi router informations.... Check and change permissions for apps.
Use alternative apps if possible from other companies.
Use search engines from other companies (Duckduckgo Startpage, Quant). You can alternate between them to spread the data.
Don't log into apps or web sites with Facebook, Google etc, just make an account for each service and don't allow access to your activity for other services.
NextDNS is great but it is only a piece of the puzzle if privacy is a concern for you. If not, you can keep giving GAFAM and to the others who sell them all your data.