You cannot directly convert a Cisco serial number into a Meraki Cloud ID yourself, the two identifiers are different.
Meraki Support can retrieve the Cloud ID for you if you provide the Cisco serial number and proof of purchase or order details
I suggest you open a support case.
https://documentation.meraki.com/Platform_Management/Troubleshooting_and_Support/Support/Ways_to_Con...
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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