Create Double NAT on BGW320-505

I need to separate part of my network from accessing the other part. I just switched to AT&T (previously on Spectrum).

My setup was modem -> gigabyte wired router -> switch -> linksys router. And this worked plug-and-play with no system changes. The devices and network on the linksys router were separate from the devices connected to the gigabyte wired router and switch.

Since the Nokia BGW320-505 is both a modem and router, I decided to start by removing my gigabyte wired router and using the BGW320-505 in place of it. All of the devices to connected to it and the switch work fine but the linksys router and it's network were no longer connected to the internet.

Should this be resolved with some 'turn it off then on again' and it will figure it out or do I need to add a pass through IP of the linksys router in the BGW320-505 admin settings?