Angelic: All the Cable/DSL modems I have come across will respond to a DHCP request and will pass the configured, (by the ISP), IP address to the responding device, (in this case, hopefully, your sonicwall). If you can tell the Sonicwall to gets it's address automatically and then restart the Sonicwall it should pick up the required IP to make the connectivity.
If you can't do this connect any workstation the ethernet connection of the modem and restart it after telling it to get all it's information automatically. The run ipconfig /all and you have the information you can manually put into the Sonicwall. If the Sonicwall can operate as a DHCP server for the internal devices, let it. Otherwise you need fixed IP's internally or fix the LAN address of the Sonicwall and set up a DHCP server with an appropriate scope internally.
