Software / Programme that will monitor up address's

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I am interested in developing a webpage-based program, app, or a suitable solution that can:

• Ping or monitor external IP addresses of routers
• Display a GUI interface where the router's status is shown as green if online and red if offline
• Send alerts via email, SMS, or pop-up notifications when a router goes offline

The goal is to monitor customers' broadband routers to ensure they are online and receive alerts when they go offline. I would appreciate suggestions on how to implement this and the associated costs.