Remember to replace "eth0" with your NIC device although usually it will be eth0.

Replace 192.168.1.1 with your default gateway, remember you can also add multiple gateways by adding a second/different default gateway.

route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *               255.255.255.0   U     1      0        0 eth0
link-local      *               255.255.0.0     U     1000   0        0 eth0
default         DD-WRT          0.0.0.0         UG    0      0        0 eth0

route del default gw 192.168.1.1 eth0

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *               255.255.255.0   U     1      0        0 eth0
link-local      *               255.255.0.0     U     1000   0        0 eth0

route add default gw 192.168.1.1 eth0

Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
192.168.1.0     *               255.255.255.0   U     1      0        0 eth0
link-local      *               255.255.0.0     U     1000   0        0 eth0
default         DD-WRT          0.0.0.0         UG    0      0        0 eth0