Unfortunately, based on my research, AWS Lightsail cannot be accessed directly using private IP addresses with regular/non-Lightsail services, because Lightsail does not use your regular VPCs.
This includes and especially affects EC2 (Application/Network) Load Balancer.
To use Lightsail with other AWS services, you can set up VPC Peering. For how much, you may ask?
There is no charge for setting up or running a VPC peering connection. Data transferred across peering connections is charged at $0.01/GB for send and receive, regardless of the Availability Zones involved.
So, depending on your priorities, your options include:
- Switch to EC2, which will give you greater flexibility and (depending on instance type) guaranteed performance. Lightsail instances use burstable instances, comparable to EC2's t3 instances.
- Install nginx inside Lightsail as reverse proxy on port 80 and/or 443, therefore expanding your options to use it directly or using an external CDN/Load balancer such as CloudFlare.
- Use Lightsail Load Balancer, and not use wildcard certificate.