There's no published API call for this.
There are two early-access API calls that can be used to calculate availability (not differentiated between device/ISP failure, but probability is that mostly it'll be ISP failure rather than device failure)...
Currently we calculate availability across multiple organizations by using this call to log status every 5 minutes, then use the date to calculate availability across the SLA period...
https://developer.cisco.com/meraki/api-v1/api-reference-early-access-api-platform-monitor-devices-st...
There is a potentially more efficient way using the newer call (I'm not migrating to it unless/until it goes general release)...
https://developer.cisco.com/meraki/api-v1/api-reference-early-access-api-platform-monitor-devices-av...