I just figured it out by myself.
Hope this could help someone, all you have to do is ssh into Speedgoat hardware, by ssh root@192.168.7.5 password: root
then the following commands:
netstat -rn (to see old default gateway, 192.168.7.5 from factory)
route delete default 192.168.7.5
route add default new_default_ip_addr
Make sure new default gateway ip address belongs to one of the ethernet interfaces subnets