Adding a default route is very simple with this command
Just replace 192.168.1.1 with the IPof your GW.
ip route add default via 192.168.1.1
How can you delete this default route if you messed it up?
It's just the opposite with "delete" instead of add
ip route del default via 192.168.1.1
This is the equivalent of the "route command's" route add default gw 192.168.1.1........