Although I can't say exactly which was the last version working (I only know
that 1.0.0-4 and 1.0.0-5 don't), I used to use without any problems my ZTE
K3806-Z USB key in the past.

Now instead since probably a couple of months if I try to connect, the network-
manager applet shows that the connection was established, but in fact no remote
address can be reached.

It is a specific problem of this USB key, since I still have no problem
connecting with a Huawei K-3765 (same SIM, same network-manager connection).

Moreover, the problematic key still works smoothly on another (openwrt) system.

I am attaching the relevant syslog part for each of the two USB keys (from
plugging in the device until connection appears to be established), plus the
output of "sudo route -n" after each.
In the logs for the ZTE key, the one not working, there are several "Invalid
mobile equipment error code: 765" messages which seem suspect, but googling for
them I only found two bugs which didn't enlighten me particularly.