http://www.josh-burkard.ch/index.php/2012/11/add-second-default-route-to-synology-nas/

for my case:

#Login Nas
SSH or Telnet
#add a static route in /etc/rc.local
route add -net 10.201.0.0 netmask 255.255.0.0 gw 10.201.213.254

#make sure automatic add the static route when nas boot.