To connect W21 to sbg7600AC2 the first step is to convert sbg7600AC2 to bridge mode.

Click on the below link for the bridge mode:

http://arris.force.com/consumers/articles/General_FAQs/SBG7600AC2-Bridge-Mode-Setup

The error code: 0000-1303 corresponds to “Router Failed to Connect to the Internet “

Solution:  Make sure the Ethernet is connected to the port labeled WAN in yellow on the mAX Router.  Additionally, try different Ethernet cables for WAN connection.  Try power cycling the modem/Wi-Fi modem and the mAX router if the connections are secure.  Disconnect all wired devices from mAX units. 

Replying to my own issue re error code 000-1303

After 4 long calls with customer support finally one of the technicians had me try to 

connect direct to computer without wireless.  It was determined that the WAN outlets on my SBG7600AC2 were not functioning.  Unit was under warranty and new unit is in transit. 

Hello AlScott

                       Since the WAN port on your device is not working you have provided for warranty replacement, but this has taken long time so we do apologies for that and we hope this replaced unit will satisfy you. Thank you for choosing Arris.