To grant full access to an Microsoft 365 Mailbox you must first connect to the remote PowerShell, and provide MailboxPermission.
AzureAD
PowerShell connect to Microsoft 365 AD
How to install Microsoft 365 AD PowerShell modules, both AzureAD and Msol and how to connect to Microsoft365 PowerShell
Install-Module is not recognized as the name of a cmdlet, function, script file, or operable program
Install-Module cmdlet by default, is not recognized on Windows 2012 R2 server. When running Install-Module on PowerShell, you will receive an error saying Install-Module : The term 'Install-Module' is not recognized as the name of a cmdlet.