LightSail Instance do not communicate with public networks

This issue where your Lightsail instance becomes unreachable from public networks while still being accessible via the browser-based SSH client is a known problem that several Lightsail users have experienced.

Based on similar cases, here are some potential causes and solutions:

1. Intermittent Network Issues: Your instance may be experiencing periodic network connectivity problems that affect external connections but not AWS internal connections (which is why browser-based SSH still works).

2. Hardware or Host-Level Issues: There might be underlying infrastructure problems in your specific availability zone.

3. Network Interface Problems: Sometimes the network interface of your instance can experience issues that cause connectivity drops.

4. Firewall or Security Configuration: There could be firewall rules or security configurations that are intermittently blocking external traffic.

Possible solutions to try:

1. Change Availability Zone: Consider creating a new instance in a different availability zone within the same region to see if the issue persists.

2. Enhanced Monitoring: Set up more detailed monitoring with Amazon CloudWatch to identify patterns or issues not visible in standard metrics.

3. Check System Logs: When you regain access after a reboot, examine all system logs (including kernel logs) for errors or warnings that might indicate connectivity issues.

4. Create a Snapshot Before Issues: Take regular snapshots so you can deploy a new instance if needed.

5. Check Firewall Settings: Verify that your instance's IPv4 firewall settings properly allow traffic on the necessary ports (80, 443, 22, etc.).

6. Resource Exhaustion Check: Although you mentioned monitoring with TOP and SAR shows everything looks fine, there might be brief resource spikes not captured in your monitoring.

If the problem persists after trying these steps, you might want to contact AWS Support for deeper investigation of potential infrastructure issues.
Sources
Lightsail instance becomes unreachable from network | AWS re:Post
lightsail suddenly cannot be accessed | AWS re:Post