I am using Linux Ubuntu 12.04 with U620L. I am able to have access to Internet but after some time and under different circumstances, the USB modem becomes unresponsive. I can't ping the DHCP IP and not able to access internet.
The only way to fix this issue is releasing the DHCP IP using dhclient -r eth1 and then renewing it with dhclient eth1,
Kernel/OS does not provide any errors/issues. Also I enabled logs on the U620L and it only shows that a device is getting DHCP IP but it does not provide anything related to communication errors.