CM1200 modem connected to AC1750 r6400v2 dropping connection

Hello! I'm working with a user whose wireless devices are getting disconnected from his wireless router (AC1750) when he plugs another device into his modem (CM1200).  Currently, the user as a network cable plugged into port one on his modem with plugs into the WAN port on the router but the issue starts when he plugs in another device, like his PC, that the wireless devices lose their connections. I read on another forum that you cannot use ports 2 to 4 but the user stated that he had no issues accessing those other ports prior to rearranging his office setup.

As part of the troubleshooting, we power cycled both the router and modem, which restored the connectivity but once we plugged in another device into one of the other available ports on the modem, the connection drops again. We logged into the web-based app, and I confirmed that he is on the latest firmware. I had the end user plug in his PC directly into the router versus the modem which kept the connection in place, but he stated that the speeds were slower.  In addition, we ran a few network commands on his PC to clear out the DNS and to renew the IP address to no avail.

I had a brief call with Netgear’s tech support who provided links to articles pertaining to port forwarding but I do not believe port forwarding is the issue since this worked prior to the user moving into a different room and the issue does not involve something on the internet attempting to access his home network.

Any assistance is greatly appreciated.