Hi
You can enable the “Override DNS Settings of All Clients” option in Admin Panel - Network - DNS.
Once enabled, all clients' unencrypted DNS requests will be redirected to the router and forwarded to the specified upstream DNS (NextDNS in your case) based on your settings.
Please note, this does not apply to encrypted DNS (DoT/DoH/DoQ).
You can try following the steps of the post shared by Admon to block the corresponding encrypted DNS services and see if these applications/IoT devices will fallback to plain DNS for redirection.
(Note that if they don’t have a fallback mechanism to unencrypted DNS, they may not function properly.)