@EnterpriseArchitect Thanks for your question. You can take advantage of Network Performance Monitoring Solution to monitor your network infrastructure which requires Log analytics agent installation. Kindly check this documentation to check if it helps with your requirement.
Monitoring OnPremise IP addresses and Network Appliances from Azure?

I wonder if anyone here can assist me to point into the right direction to create simple OutOfBand monitoring solution.
I have already established the ExpressRoute circuit from my Data centre to Azure Regional datacenter.
Requirement: Monitor the OnPremise Network appliances (Gateways, Firewalls and other services IP addresses) from Azure, then send an email alert or trigger alert via PagerDuty.
https://www.pagerduty.com/platform/
However, as per this documentation https://learn.microsoft.com/en-us/azure/azure-monitor/platform/agents-overview
I cannot find any way to monitor just a few select IP addresses of network appliances, since it requires Agent install.
I also cannot configure Pingdom service to test my network appliances IP address either.
https://tools.pingdom.com/
Thanks in advance.