LTE on Ubuntu 12.04 with AirCard 340U?

I'm using Ubuntu 12.04 (64 bit) on my laptop (DELL Latitude E6410). I'm trying to access AT&T LTE network using LTE dongle AirCard 340U but it seems like the Ubuntu OS can only use ppp0 interface to access UMTS by the LTE dongle. I have tried this dongle on Windows 7 and it works well (I can access LTE network so it's not the problem of SIM card).

I also looked at these two links for setting up driver support: http://kb.netgear.com/app/answers/detail/a_id/22827, http://kb.netgear.com/app/answers/detail/a_id/22869. My Ubuntu has both sierra and sierra_net kernel modules. This module is not automatically loaded when I plug in the dongle, so I need to manually attach the driver ('sierra' or 'option'). But currently ppp0 configured by ubuntu network manager does not support LTE, only 4G (HSPA) or 3G (UMTS), and the network throughput is very low using this method.

Since I must make the dongle work for LTE on Linux, it would be great if anyone with similar experience can provide any useful instructions. Thank you so much!