The APN (Access Point Name) allows the IXGW-TGW or IXGW-TGW-B to use its built in SIM card. The gateway can only use the included SIM card, and the APN settings will always be the same for the specific model.
IXGW-TGW: iot0723.com.attz
IXGW-TGW-B: mnet.bell.ca.ioe
For full instructions, including where to enter the APN address, see this article: https://know.aiphone.com/instructions-ixg-support-tool/ixgw-tgw-gateway-settings-and-telephone-registration-guide