To use a MySQL instance as a Read Replica, it must meet the following criteria:
- The instance must be a standalone MySQL server. 
- The instance must be MySQL 8.0.23, or higher. 
- The instance must not have any unmanaged replication channels configured. 
- The instance must use the same credentials as those used to manage the Cluster. 
        When an instance is added as a Read Replica, the AdminAPI runs
        the same compatibility tests on it as it does an instance being
        added to a Cluster. The same variables are checked, and so on.
        As such, it is recommended you run
        dba.checkInstanceConfiguration() or
        dba.configureInstance() before attempting to
        create a Read Replica.