Perform the following:
iS5comm(config)# ip dhcp option 3 ip <IP Address>
iS5comm(config)# ip dhcp option 3 ip 12.0.0.2
To configure a default router or gateway, configure the option parameter as 3 and assign an IP address.
For a list of DHCP options, refer to RFC 2132 “DHCP Options and BOOTP Vendor Extensions”