Connecting ADSL2+ Modem Router to Wireless DSL Gateway

I am having trouble connecting my Wireless Gateway to my ADSL2+ Modem Router.

The Modem Router is a Netcomm NB5 ADSL2+ Modem Router(IP: 192.168.1.1)
The Wireless Gateway is a 3Com OfficeConnect Wireless 11g Cable/DSL Gateway (IP: 192.168.1.2)

I suspect it is something to do with the DHCP Settings. On both the Netcomm and 3Com devices they currently both act as a DHCP Server.

Which one should be and how should I configure their IP Addresses?

How do I set the 3Com as the default gateway?

Any suggestions would be greatly appreciated. Thanks

Nick