I was using a "ZTE" 3G Mobile Broadband to connect with the Internet. It had some hardware issues, and I bought a new "Prolink" 3G Mobile Broadband. When I try to connect it, Ubuntu shows a "Network Disconnected" message.(Then the device turns off)
How ever I found a temporary solution.
I ran the following command on Terminal,
sudo killall modem-manager
Then I unplugged the Mobile Broadband device, and plugged it again. Few seconds later, I successfully connected to the Internet from Network Manager.
Now I have to do this every time Ubuntu starts.
(I tried a command in a forum (which I cannot remember) to view the current Network device. It showed the detail of my OLD mobile broadband. )