Open Port Monitoring

Hi Everyone,

I am hoping that someone can point me in the right direction. I am looking for free software that can help us monitor specific ports on our network. We have approximately 100 routers around the country that are connected to NVRs and Alarm Notification Systems. As these systems are remote and run on battery / solar mostly, we need to keep an eye on if these systems go down and we are currently doing it by manually port scanning 5 ports on our internal VPN network,

I am looking for some simple software that we can put our IP addresses in, define the ports we want it to watch and send an email alert if a port has been down for say longer than 10 minutes.

Does anyone have any specific recommendations?

Thanks in advance.