Reference: http://kb.zyxel.com/KB/searchArticle!gwsViewDetail.action?articleOid=013302&lang=EN
Step :
You can use schedule run by the following steps:
(1) Make sure the date and time is properly set on USG20.
(2) Upload a shell script to the device.
a. The contents of .zysh are:
configure terminal
reboot
b. save this file as "reboot_device.zysh"
c. upload this file to shell scrip page. (Maintenance > File Manager > Shell Scrip)
(3a) Use CLI command to run the schedule everyday:
Router(config)# schedule-run 1 reboot_device.zysh daily 03:00
Router(config)# schedule-run 1 reboot_device.zysh weekly 03:00 sun