System DSN for SQL Server to connect through ODBC with for a specific port
To specify a non-default port number, add a comma and the port number after the server name. For example, with a port number of 12345:

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\ODBC\ODBC.INI\SQLtest]
"Driver"="C:\\WINDOWS\\system32\\SQLSRV32.dll"
"Description"="SQLtest"
"Server"="BLR002,12345"
"Database"="Maint"
"LastUser"="sa"


If the SQL instance is a named instance (named MAINT) like the DSN in your question shows, you should be able to connect using with your original DSN specification instead of port as long as the SQL Server Browser service is running on the server, UDP port 1434 allowed through the firewall on the server, and the instance port allowed as well.