How to automate running the dhclient for wwan0?

Question

in Linux Mint 17 I have to run sudo dhclient every time I connect and disconnect to/from the Internet using a mobile broadband dongle, because otherwise the connection is declared active but when I try to open a web page my browsers freeze in the "resolving host" state (and the email client too).

I thought the problem was very similar to the one mentioned in this question, so I edited "/etc/network/interfaces" adding the last two lines:

# interfaces(5) file used by ifup(8) and ifdown(8) auto lo iface lo inet loopback auto wwan0 iface wwan0 inet dhcp

then I also run sudo /etc/init.d/networking restart but nothing changes (I rather get these messages: "Waiting for network configuration...", "Waiting up to 60 more seconds for network configuration...", "Booting system without full network configuration...").

How should I proceed? Thanks a lot.