On the Organisation-> MDM ("../manage/organization/pcc") page, the "public key cert" is unique per page visit. When you click "Add new" you will then be given access to download this.
After downloading the (*.pem) token, you MUST NOT change page, refresh or delete an old server prior to uploading this to Apple and then uploading the Service Token (*.p7m file) from Apple to the page.
If you did change page or refresh this will mean you can create a new Apple MDM Server, but it will then fail when uploading it back to Meraki with the error "DEP token cannot be empty" | "failed to parse service token".
This error should ideally say "Please download a new public cert key and create a new Apple MDM Server. Please do not leave or refresh this page prior to uploading the Apple MDM Server Token.". ---