I tried disabling NAT. That simply made things unusable. Had to reset the device to get it working again.
In the meantime, I received a reply from Zoom. Without admitting that the modem/router can't be configured as a bridge, I was advised to purchase just a "modem" that would act in bridge mode. So, I did that. I picked up a Cisco DPC3008. After a bit of fiddling and a couple of calls to Comcast, I've got it working with my NetGate pfSense router.
Thanks for all the input. I'm sure you'll be hearing more from me as I try to figure out how to configure the firewall, port forwarding and failover stuff. I'll try not to be a pest, though.
Cheers,
Dave