Try this (copied from a Co-Pilot Bing Search)

 To configure your AT&T BGW210-700 gateway in IP passthrough mode, follow these steps:

Access the Gateway Settings:

Open a web browser on a computer directly connected to the Arris BGW210-700. Enter 192.168.1.254 in the browser’s address bar.

Navigate to IP Passthrough:

Click on the IP Passthrough tab to configure the following settings. Configure IP Passthrough:

Change the Allocation Mode to Passthrough. Enter the Passthrough Fixed MAC Address of your router’s WAN port (usually found on a sticker on the router itself). Save the changes.

Additional Notes: After setting up IP passthrough, you will still access the gateway settings at its default IP address (192.168.1.254), just as you did before enabling passthrough.

Ensure that your router is set to automatically receive an IP address from the modem (usually under the WAN settings).

Turn off Wi-Fi on the modem and consider disabling any firewall features on the modem (if your router handles firewall functions).

Remember to reboot the modem first, and then your router.