How do I get the IP address via AT commands?

Use the +WIPC command:

Display IP Address +WIPC
Description: This command is used to display the IP address that is currently assigned to the module. An open PPP session is required.
Values: None.
Syntax: Command syntax: AT+WIPC

Command Possible responses
AT+WIPC +WIPC: 68.25.209.28
OK
Note: Display current IP address. Note: IP address displayed.