The modem I have is Sierra Wireless Air Prime MC7453 Mini PCI Express Module. The carrier I tested is AT&T.
Before running on Linux, I had to install the card on a regular Laptop that supports 4G card, and run Windows 7 on the laptop, install all the drivers and the Sierra Wireless Connect Manager to try out the card. This confirmed that the card has the latest firmware, and the carrier setting is correct, etc.
Next I run it on Linux. Instead of using the Gobi driver from Sierra Wireless, I used the qmi_wwan driver in the latest Linux (after kernel version 3.14). An user land utility is also needed. libqmi seems to be popular, but it depends on a fairly new version of glib, which itself depends on a bunch of libraries. It becomes a hassle to compile everything. Then I discovered uqmi, a small utility developed and used by the openwrt project. It's much more lightweight, and seems to be pretty mature. I was able to get my modem to connect and ping with the following commands.
uqmi -s -d "$device" --get-pin-status
uqmi -s -d "$device" --get-serving-system
uqmi -s -d "$device" --set-data-format 802.3
uqmi -s -d "$device" --wda-set-data-format 802.3
uqmi -s -d "$device" --set-network-modes "$modes"
wds=`uqmi -s -d "$device" --get-client-id wds`
uqmi -s -d "$device" --set-client-id wds,"$wds" --start-network "ISP.CINGULAR" --autoconnect
uqmi -s -d "$device" --get-signal-info
uqmi -s -d "$device" --get-data-status
Now I can ping
uqmi -s -d "$device" --stop-network 0xffffffff --autoconnect > /dev/null
 The home page of uqmi: http://nbd.name/gitweb.cgi?p=uqmi.git;a=summary
 A list of useful commands of how to use uqmi: http://trac.gateworks.com/wiki/modem
 The official Openwrt qmi script that uses uqmi: https://dev.openwrt.org/browser/trunk/package/network/utils/uqmi/files/lib/netifd/proto/qmi.sh
 Another useful link: https://lists.openwrt.org/pipermail/openwrt-devel/2014-April/025046.html