You could call /usr/sbin/ubnt-add-connected.pl every 5 minutes directly, but that script is quite CPU heavy to be executed so often (even when not needed). To execute the script on ER-L takes 2.7 seconds (and may take longer depends on how many routes you have ... I have only 6).
My script /config/scripts/fix-add-connected.sh is executed every 5 minutes but it checks first whether the routes are missing (which is very light check) and only calls /usr/sbin/ubnt-add-connected.pl when needed (when the routes are missing). To execute my script on ER-L (when no routes need to be added) takes only 0.05 seconds.