Hi, Lightsail managed it's own network. If you want to connect Lightsail to RDS need to user VPC peering.
Please refer below link. https://aws.amazon.com/lightsail/faq/ Jump to question : Q: How can I connect Lightsail instances to other resources in my AWS account?
The alternate way is instead RDS use lightsail managed database. https://aws.amazon.com/lightsail/pricing/ check jump to "Managed databases" section.
Thanks and Regards.
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