I had the same problem on my Debian Jessie : when connecting the Mod, I could not reach external websites anymore.
What happens is that NetworkManager overwrites /etc/resolv.conf with
Basically, DNS queries are now routed to the Mod (!) which causes them to fail.
The solution, is to run
from the command line as root, then to click on the interface
corresponding to the mod-duo, to select the ipv4 tab and to set the
correct DNS address (192.168.0.1 on my local network).
Also set Automatic addresses (DHCP) only instead of Automatic addresses (DHCP)
Then click apply and the problem should go away.
I hope this helps