DNS-over-TLS got introduced natively in Android 9 (“Pie”), so not sure if the Nextdns App offers its own DoT/DoH implementation or is a interface to the custom settings. Their thread linking back here - https://help.nextdns.io/t/x2hlnta/nextdns-does-not-work-on-android-8-1-e-os. A user suggesting Nebulo (com.frostnerd.smokescreen) and adding your own NextDNS link there is an avenue you can test.