I did not find the information I am looking for in your docs.
That is exactly the problem: the device cannot access the Internet, because the access is restricted by the firewall, it does allow outgoing connections to particular IPs only. Therefore I have to configure the firewall to give the Slate access to the Internet, see my original question.
You have answered this question in this forum already with respect to the US servers: What are the GoodCloud IP address ranges?, but I need to connect to the European servers. And for me it is also important which incoming traffic from the cloud to the device has to be allowed in the firewall.