Never worked for me either. Pretty sure it's not currently possible on the US R11e LTE modem (but works on the Int'l one)
Another posting was requesting it as a feature:
Can we please have manual band control/locking for the US modem? I have dozens of SXTs deployed, and they are jumping into Band 12 from Band 2, and Band 12 is over-provisioned and near useless in my area. I need the ability to force Band 2 or 4, and the modem doesn't even support any of the same AT commands that are listed in the wiki. Support tells me that the wiki is for the international modem only. When support doesn't ignore me.
Can you please add this feature? It would make me not have to move to another hardware solution. I just can't have a device do 90Mbps down one day, and 10Mbps down the next, and generally bounce around based on weather and leaves.