+1 Karma to Mikrotik support people.
Opened a ticket and they pointed me to an old topic on this forum with a new addition:
http://forum.mikrotik.com/t/rb750up-3g-usb-freezes-completely/54120/1
In this topic there’s a point that Sierra Wireless cards do nothing on ATH command (used by Routerboard to reset 3G card), and that AT!GRESET is the correct way to reset the card (Right now it seems to work [at least for some hours]). Huawei cards seems to work fine with AT+CHUP (not tested).
You can see a script for rebooting in case the test detected something wrong.
Hope this helps.