Since you are working on MSP, in order to get the list of monitors you need to pass the zaaid. If you are working on getting list monitors for N customers you need to make N requests.
"By default, all API requests executed as part of the MSP account operation will use the standard header parameters. You do not have to generate any additional cookie headers to initiate an API request. However, If you initiate an API request for an unauthorized MSP operation, the following error message will be shown "You are not authorized to perform this operation.""
For this api www.site24x7.com/help/api/#list-monitor pass in the zaaid of the customer as a cookie parameter in the header.
$ curl https://www.site24x7.com/api/monitors
-H "Content-Type: application/json;charset=UTF-8"
-H "Accept: application/json; version=2.0"
-H "Authorization: Zoho-oauthtoken <authtoken>"
-H "Cookie: zaaid=<actual zaaid>"
Please read the msp api help doc for full reference with examples.
We have separate APIs for getting global MSP account data across all Customer Accounts. That's the reason you're able to get the current status across customers.