There is good manual in openwrt site:
http://wiki.openwrt.org/doc/recipes/3gdongleI'm using 3g modem with carambola, and haven't noticed such behaviour (Huawey). Maybe your device driver fails to enumerate correctly. You could check if /dev/ttyUSBx device file is removed once device is disconnected. If not, maybe you can add 'rm /dev/ttyUSB*' command to disconnect hook. Anyway, I *think* I remember someone having this problem, but unfortunately can't remember the solution
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