Hello Brian, Welcome to MS Q&A
For SQL Managed Instance, you cannot use IP-based firewall rules to restrict access to the public endpoint. The document results indicate that IP-based firewall rules apply to Azure SQL Database and Azure Synapse Analytics, but not to SQL Managed Instance.
For SQL Managed Instance, you should consider using private endpoints or virtual network firewall rules to restrict access. This allows you to control access from specific virtual networks within the Azure boundary.
For more information on connecting to Azure SQL Managed Instance and configuring network access, please refer to the following documentation:
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Deepanshu