Problems with UMTS modems and weak signal

[Martin Bammer <mrb74 gmx at>]

When I connect to the internet with a UMTS modem (Huawei E156G) I have
problems with the connection when signal is sometimes to weak (e.g. when
travelling with a train).
When signal is too weak the connection gets lost. (Thats normal and ok).
But when signal strength is strong enough again to reconnect network
manager does nothing. It should automatically reconnect like it does
with wireless connections.
Second problem is that I also can't reconnect manually. When connection
gets lost the "Disconnect" menu entry disappears. So I only can click on
the menu entry with the name of the network provider. But when I click
on it nothing happens. The expected reaction of nm should be to
reconnect. What I have to do to be able to reconnect is to unplug the
umts modem, replugin it again and click on the network provider menu
entry to reconnect. That's very uncomfortable.
Is there a way to solve this problem?