The easiest way to set Quad9 on your entire network is via your router settings. If you'd prefer to set Quad9 on a single system, please follow the steps below to configure Synology NAS to use Quad9.
Screenshots and instructions graciously provided by https://mariushosting.com
1. Go to Control Panel / Network / General.
2. Check “Manually configure DNS server”
3. Preferred DNS Server: 126.96.36.199
4. Alternative DNS Server: 188.8.131.52
5. Click "Apply".