Sierra Wireless 340u:

1. Connect the device to a Windows computer
   

2. Activate
   

3. Run AC340U_Linux_Patch executable to put in IP passthrough mode. (Download here: ac340u_patch_v4.zip)
   

4. Wait for the tool to complete.
   

5. Set to 4G Sierra Wireless in SimpleWAN Dashboard for WAN or WAN2 (Follow Normal 4G Setup procedure from here).
   

6. AT&T has made changes to the latest firmware upgrade for this device which removes all support for this. Please be sure to NOT upgrade those devices if you want to use them with the SimpleWAN device.
   

NOTE: On  AT&T devices, even when they are connected in this IP Passthrough mode the display says “Ready to Connect”. After you apply Failover, you'll need to reboot your SimpleWAN Appliance. If you do not see the speed test results after 10 mins please reboot the unit to initialize a speed test on both the primary and/or secondary connections. Double-check or repeat the steps above if needed.

 When using failover, you need to use external DNS servers Like Google otherwise failover won't kick in.