Getting NETGEAR Cable Modem Gateway CG3000D-2 to coexist with FRITZ!Box 7270

Hi Experts,

I have been running a FRITZ!Box 7270 as an ADSL modem, router etc.

I have just added a NETGEAR Cable Modem Gateway CG3000D-2 (because I have switched from ADSL to cable Broadband).
It's on a different subnet (192.168.0.x) to the FRITZ!Box (192.168.1.x)

These are the LAN Settings for the NETGEAR:
LAN IP Address:       192.168.0.1
Subnet Mask: 255.255.255.       
DHCP Server:  No  (was Yes, but I want to change it to No, so that the FRITZ!Box does DHCP)

I have tried changing the settings to:

LAN IP Address:       192.168.1.254
Subnet Mask: 255.255.255.       
DHCP Server:  No

But when I click the Apply button, I get this error:

"LAN IP address and Guest LAN IP can't be in the same subnet"

What's the "Guest LAN IP"?

Regards,
Leigh