To access the SQL database managed instance from an on-premises location, you must make a site-to-site connection between the application and the Azure SQL managed instance virtual network. For information about the choices you have for making this connection, see communicating with on-premises resources in the Microsoft documentation.
For the site-to-site VPN and Azure ExpressRoute methods of connection, if you established an on-premises-to-Azure connection successfully and you can't establish connection to the managed instance, verify that your firewall has opened outbound connections on SQL port 1433 as well as on ports 11000-12000 for redirection from on-premises proxies.