To get the IP address file a ticket with the question with Mendix support, they can provide this information. If SFTP is needed, you'll indeed need a certificate for this and could use the (s)FTP module from the app store (upgrade needed from version MX5.81 though, for the module)
What if you just opened up cmd.exe on Windows (Windows Key + R then type in cmd.exe click OK)
Then just ping your Mx app by typing in
Pinging it should reveal the ip your app is hosted at unless there's a strange firewall preventing this or if it's IPv6 only.
You can also use command prompt to perform a traceroute by simply,
This will reveal each hop and ultimately land you at the location of the server. I personally use traceroute to learn about where my datacenters are located how how my packets are being routed. It can also reveal interesting things like ip's of load balancers. For instance, TIL mendixcloud.net is using Rackspace! Really awesome!