Hello Aliengray,
In the English Ubuntu Forum there is some talk of the same issue too. one topic heading is called:
"How to lock mobile broadband on 3G network only ?"
And it is suggested to use WVdialer as a work around with the following commands
AT^SYSCFG=13,1,3FFFFFFF,2,4 # GPRS only
AT^SYSCFG=2,1,3FFFFFFF,2,4 # GPRS prefered
AT^SYSCFG=14,2,3FFFFFFF,2,4 # 3G ONLY
AT^SYSCFG=2,2,3FFFFFFF,2,4 # 3G prefered
WVdialer wasn't working for me so I wanted to try something better. But maybe these commands do work for our stick?