If you send me a CC debug log I might be able to work out where things are going wrong. See
How do I get help? in our FAQ for instructions.
It is possible that your device is confused about the internet name service (DNS). Try turning it all the way off (power off) then on. Verify that you can see an internet web page using the device's browser.
Another thing to try (if you haven't already) is to reset the cloud connection then try to connect again. Go do CC's Settings / Connecting to calibre / Cloud Connection and tap Reset cloud connection.