I am only able to connect to a MySQL Aurora cluster's reader endpoint and read replica instance endpoints. However, connection timeouts occur when connecting to the cluster's writer endpoint and the primary writer instance's endpoint.

Additional details:

• Using MySQL Workbench
• Using Cluster's master username/password
• Access to VPC through VPN
• Cluster SG has VPN IP range whitelisted
• Traffic allowed through subnet NACL's
• Cluster instances in private subnets

Any thoughts what might be causing this issue?