means that your node is not available from outside. Perhaps your external IP has changed, but myqnapcloud DDNS name is not updated.
You can setup uptimerobot.com to monitor your external address, you need to use the same address and port as your node for check.
You can also see when your node was not available with these scripts:
You can use other DDNS provider and setup update of the DDNS hostname on your router, it could be faster than myqnapcloud