Yes in this case you need to follow the steps mentioned on the below article to enable Bridge mode on the modem.

http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Bridge-Mode-Setup

What if the Netgear Mesh Router is already using that IP?  Can I directly connect to the modem?

If you’re on a residential account for your Internet, you’re subscribed to one IP.  Only one device with the IP will have Internet access and other devices will not get an IP unless you subscribe to additional IP (if your ISP allows it).