USB Modem, ttyUSB* problems

Hello everybody!

SOME BACKGROUND
Ubuntu 12.10

juan@ktdn:~$ uname -a
Linux ktdn 3.5.0-17-generic #28-Ubuntu SMP Tue Oct 9 19:32:08 UTC 2012 i686 i686 i686 GNU/Linux
juan@ktdn:~$

juan@ktdn:~$ lsusb
Bus 001 Device 002: ID 058f:6254 Alcor Micro Corp. USB Hub
Bus 001 Device 010: ID 19d2:0124 ZTE WCDMA Technologies MSM
Bus 001 Device 004: ID 04f2:b012 Chicony Electronics Co., Ltd 1.3 MPixel UVC Webcam
Bus 004 Device 002: ID 0b05:1712 ASUSTek Computer, Inc. BT-183 Bluetooth 2.0+EDR adapter
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 005: ID 0951:1624 Kingston Technology DataTraveler G2 4GB Pen Drive
Bus 001 Device 008: ID 1c4f:0002 SiGma Micro Keyboard TRACER Gamma Ivory
Bus 001 Device 007: ID 13ee:0003 MosArt Optical Mouse
juan@ktdn:~$

juan@ktdn:~$ nmcli dev list iface ttyUSB4
GENERAL.DEVICE: ttyUSB4
GENERAL.TYPE: gsm
GENERAL.VENDOR: ZTE,Incorporated
GENERAL.PRODUCT: ZTE WCDMA Technologies MSM
GENERAL.DRIVER: option1
GENERAL.DRIVER-VERSION:
GENERAL.FIRMWARE-VERSION:
GENERAL.HWADDR: (unknown)
GENERAL.STATE: 100 (connected)
GENERAL.REASON: 0 (No reason given)
GENERAL.UDI: /org/freedesktop/ModemManager/Modems/0
GENERAL.IP-IFACE: ppp0
GENERAL.NM-MANAGED: yes
GENERAL.AUTOCONNECT: yes
GENERAL.FIRMWARE-MISSING: no
GENERAL.CONNECTION: /org/freedesktop/NetworkManager/ActiveConnection/0
CAPABILITIES.CARRIER-DETECT: no
CAPABILITIES.SPEED: unknown
IP4.ADDRESS[1]: ip = 213.99.125.133/32, gw = 10.64.64.64
IP4.DNS[1]: 80.58.61.250
IP4.DNS[2]: 80.58.61.254
juan@ktdn:~$

THE PROBLEM
Most of the time my ZTE gets attached to ttyUSB2 when I plug it, and this device does not work. I usually have to unplug it,
then plug it again, and even shut down the laptop until it is (mysteriously) tied to ttyUSB4, and then it works.

I am getting tired of this protocol and my question is:
is there any way to disconnect from ttyUSB2 and connect to ttyUSB4 once it has been assigned by the modem-manager?

I tried (with the modem attached to ttyUSB2):
nmcli con up id <my conn id> iface ttyUSB4

but nmcli complaints about an unsuitable device.

I'll appreciate some help.

Thanks in advance!