G54 is a gateway that is combination of modem and router so you need to enable bridge mode on the on the modem before setting up the max router to G54. Once the bridge mode is enabled you can connect router WAN to modem LAN port. Follow the steps mentioned on the below article to setup the router.

http://arris.force.com/consumers/articles/General_FAQs/G54-Bridge-Mode-Setup

http://arris.force.com/consumers/articles/General_FAQs/mAX-W30-W31-Wi-Fi-Cable-Modem-Internet-Setup

Seems a waste to buy a $600 router only to use it as a modem. Much cheaper options. 

I’d probably try and setup the W133 in bridge/access point mode instead. 

https://arris.my.salesforce-sites.com/consumers/articles/General_FAQs/mAX-Routers-Bridge-Mode-Setup/?l=en_US&fs=RelatedArticle

That would allow you to keep the G54 as a router and still use the W133. Just make sure the G54 and the W133 are using different wireless channels and ssid’s to prevent interference.