ssh to the device.
then add your route to this file:

echo "route add -net x.x.x.x gw y.y.y.y" >> /etc/persistent/rc.poststart

try running the script manually to see if it works:

./etc/persistent/rc.poststart

reboot;

cfgmtd -w -p /etc/

By: Mehdi Sadighian
Contact: mehdi.sadighian@hotmail.com
TAG: ubnt,ubiquiti,route,persistent route,static route