This applies as of PRTG 24
Can the SSL sensor or another send warning alerts for pending expirations (in x number of days) and expired certs?
Please have a look at the following:
You are going to need one SSL Certificate sensor per certificate with this approach, unless there is a device-specific approach that can be used to collect these statistics directly from the Fortigate equipment that can be implemented with custom sensors.
Disclaimer:
The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.