just to be sure whether you are using a static IP or getting from the DHCP(Mobile Broadband)
cat /var/log/syslog and check for something like below..
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 6
DHCPOFFER from 10.100.1.254
DHCPREQUEST on eth0 to 255.255.255.255 port 67
DHCPACK from 10.100.1.254
if you have something similar to above then you are using DHCP else you've a IP address configured and need to remove it.
I think you also need to check whether you are getting the correct IP address, to do this use.
to check the Gateway use
then ping to the gateway IP address to check the connectivity if everything is fine then you should be able to ping to your gateway IP address.