Option is available in GUI only in the latest software (9.9, 9.10), otherwise you need to use SSH. Which is actually pretty streightforward. Log in, send the command, reboot.
I have seen couple times this kind of problems exactly with 9.7.2, probably there may be a bug with discovery. Anyway, set director IP works always... Also probably you want to upgrade ZD to later version (if you don't have legacy APs).