Can we monitor the status of external devices such as routers, sensors, etc in cloudwatch ?

You can use Lambda and CloudWatch to create custom monitors like pinging a router interface. You can schedule the Lambda to run every so often and send the response as a custom metric to CloudWatch.

• https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/publishingMetrics.html
• https://docs.aws.amazon.com/AmazonCloudWatch/latest/events/RunLambdaSchedule.html

You can also look at CloudWatch Synthetics to create custom canaries that can monitor your applications and endpoints

• https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/CloudWatch_Synthetics_Canaries.html