can ssh, but not connect, how to troubleshoot?

[mrkagelui]

what I've done

provisioned an algo server on AWS lightsail, all is good, but the next day it always stops working.

what I've tried

1. with wireguard and previously working tunnels, connecting with various means including through a router and without, on different devices, all failed. (none of the network traffic can go through when connected to that VPN);
2. firewall of the algo server seems correctly provisioned, port 51820 is open;
3. I can ssh to it, installed net-tools and verified that the "mtu" is way above 1500 on all interfaces;
4. reboot the server via lightsail UI, not working afterwards.

ask

how do I troubleshoot this further? kindly let me know if more information is needed 🙇