L2TP connection setup is tracked in the log. Check the log (Log > View page of the SonicWall's user interface) for messages to assist in determining the problem with L2TP initialization. There are log messages regarding:
L2TP Connection Initiated
L2TP Session or Tunnel Negotiation Started
L2TP PPP Authentication Failed
Attempt the following troubleshooting steps if you cannot establish an initial connection to your ISP:
Ensure proper power connections. Ensure the correct SonicWall power supply is being used if external (5V, 2.4A rating: except the TZ150 which is 12V, 1.66A).
Check the WAN link light. If there is no link, check the cables to the DSL/Cable modem and verify the ports are OK.
Try rebooting the DSL/Cable modem and the SonicWall. (Some modems require reboot whenever SonicWall reboots.
Ensure there is no DSL/Cable modem or line problem. (no L2TP negotiations)
Ensure the username and password are correct (authentication success or failure).
Verify you get a proper IP lease and can ping the ISP gateway from the SonicWall. If not, contact your ISP.
Try connecting directly with another host or PC. If this does not work, contact your ISP.
Ask your ISP if they have measures that prevent you from switching equipment. The ISP may lock IP assignments to MAC addresses.