I was previously running ubuntu 12.04 and upgraded to ubuntu 12.10. Previously on 12.04 when I removed my wired ethernet connection and plugged in my 3G modem I was immediately able to browse the internet. However now on 12.10 I am unable to do so, even though the modem shows that it is connected when I go to Firefox or Chrome it displays an error. In order for my 3G to work I need to restart my machine (without the ethernet cable plugged in) and then plug in the 3G modem. It then works.

Please help or advise what I can do.