The command that is failing is the one that installs the key that is used to sign the MongoDB packages, so that
apt-get can determine whether the packages are genuine.
You could try skipping installation of the key and adding
--allow-unauthenticated when you run
apt-get to install the packages. See How do I bypass/ignore the gpg signature checks of apt? for more detail.
But ideally, you need to figure out why you cannot connect to the key server to install the key.
Firstly, you can check your Internet connection and DNS by visiting http://keyserver.ubuntu.com/ in a browser. If your Internet connection is working you will see a simple page entitled SKS OpenPGP Public Key Server. This shows that your computer can look up and connect to the key server.
If this is the case, then it is likely that the problem is a firewall or proxy server restricting your access to the specific port that
apt-key wants to connect to. See No gpg key from behind a firewall for details for how to allow
apt-key to connect via a proxy server.