1) check the firmware version on your Sierra Wireless / Netgear modem; this can especially help if using older, discontinued models like the Sierra Wireless 313U
2) check the Sierra Wireless AirCard Watcher (or carrier-branded version, such as AT&T AllAccess) to make sure that the modem is set to Automatically Connect.
3) there have been instances where Sierra Wireless modems will sometimes fail to connect using the Auto or built-in preset configuration, or the Serial/GSM configuration, but connect in Composite mode with the same settings. The opposite has also been observed (Composite does not work, but Serial/GSM configuration does work).
This behavior has been seen on the same cell carrier using different settings in different regions (aka AT&T within the US, or Rogers in different parts of Canada), or between carriers in the same country (Bell Mobility vs. Rogers in Canada, etc.) so it may take some experimentation to see what will work for a given carier in a specific area.
It's even been observed where the same carrier seems to require different configuration methods in a different part of the country, or when in an LTE-capable area versus older (HSPA or 3G) areas.
For use with the Cube, Brik, Bond or Link you can try the following:
"GSM" Configuration Mode:
Older AT&T accounts might use an APN of isp.cingular but new ones typically use broadband.
"Composite" configuration mode: