I am able to connect to your sandbox cluster. I believe it must be a network/firewall related issue at your end.
Can you please make sure you are able to ping the cluster by running the following command in your terminal ?
And also please verify that port 27017 is not blocked by clicking http://portquiz.net:27017
On some occasions, even though the above mentioned tests passes, network is still a problem. So please make sure you are not connected to any VPN and firewall/antivirus is disabled. You can also try to connect from some other network such as your home network .
Hope it helps!
If the issue still persists please feel free to get back to us. Happy Learning
Curriculum Support Engineer