I use these NVRs - Buy Foscam FN3109H 9 Channel IP Camera NVR | Foscam UK
The have 4 dry-contact inputs on the back that you can assign to camera channels (so four cameras you can trigger for external alarms).
Any dry-contact relay can be used (Z-Wave / ZigBee) but I use KMtronic devices for this - https://www.kmtronic.com/
One of their 8 channel relays lets me "ping" 4 cameras on each of my two NVRs .... when playing back you can search all recordings or just alarm recordings.
