Some AT&T 340U modems have a flaw that results in intermittent USB communication problems. Also if the “auto-connect” feature of the AT&T 340U modem is enabled it may cause problems when the modem is used with SonicWall products. In addition, the AT&T 340U modem must be provisioned with an “i2gold” APN plan through AT&T.
Other names used for this modem include “AT&T Beam or 340U”, “Netgear Beam or 340U” and “Sierra Wireless Beam or 340U”. Also “340U”, “340” and “AC340U” are used interchangeably. “AT&T 340U” is used in this article.
If you are experiencing problems with the AT&T 340U, follow these troubleshooting steps:
Use an alternative model modem if possible. Due to the known issues with this modem model, SonicWall suggests customers who want the most reliability and performance or are having trouble with a 340U use a different modem model. Please check the list of current supported AT&T USB modems here: https://www.sonicwall.com/en-us/support/knowledge-base/170505473051240
USB extension cable. This suggestion should be considered a work-around until the AT&T 340U modem is replaced with a newer model. The modem may have an internal USB communication problem. It has been observed that adding a quality 3 foot USB extension cable between the firewall and modem may reduce or eliminate this problem. SonicWall suggests the AmazonBasics 3.3 Foot USB 2.0 Extension cable part number B00NH11R3I : https://www.amazon.com/AmazonBasics-Extension-Cable-Male-Female/dp/B00NH11R3I
Verify auto-connect feature is disabled. The “auto-connect” feature may be enabled on the modem. To turn-off the auto-connect feature will require a Windows PC. On a Windows PC download the AT&T Global Network Client for Windows: https://www.corp.att.com/agnc/windows/ . Run the AT&T Global Network Client on the Windows PC. Now connect the AT&T 340U modem to the Windows PC. Wait for the AT&T Global Network Client to detect the modem is connected. If the modem automatically connects, click the “Disconnect” button in the AT&T Global Network Client. Now in the AT&T Global Network Client navigate to Settings | Mobile | Automatic Mobile Connection and verify that the “Disable Automatic Connection” option is checked. Try the modem again with the SonicWall firewall.