Crontab does not inherit the root PATH so any executable called from crontab needs to either be called using its full path (including paths within sub-scripts) or you need to set the path at the beginning of your crontab file before any executables are called). Have a look at /etc/cron.d/0hourly as an example.
Can I ask what is the /install/script/dyndns.sh doing? Did you know you get a free DDNS name with your ClearOS installation? Also ddclient is a good live DDNS updater if you don't want to use your poweredbyclear.com DDNS name.
Lastly, if you don't want to run from crontab, you can also run your cron jobs from /etc/cron.d just by dropping a file in there.