Is the instance setup as an FCI (failover cluster instance) - or AlwaysOn?
When installing an instance for FCI - the clustered resource will have an IP address and domain name assigned in DNS. That name is what you reference when accessing that instance in the cluster. If you are installing standalone instances that are built as AlwaysOn availability groups - then you need to create an IP address and name in DNS and add that as a listener in the AlwaysOn configuration. You then reference the instance using the listener name or IP address.
For either configuration - you also need to have SQL Browser running and not disabled.
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