Connection Pro setup.

Our setup is: Comcast Modem plugged into switch. Server 2012 does DHCP and DNS. The connection Pro is plugged into the Comcast modem. Will this work as setup or do we need  manually switch the Lan wire or buy a pricy fall-over router to make this work? Got an address, but no default gateway when I tried plugging directly into the Connection Pro. Could not ping anything. Backup is showing on standby in the status page. Thanks.