In the web interface, go to Administration/Scheduler (Custom 1 to 5)
Turn WiFi on:
wl -i ethX radio on
Turn WiFi off:
wl -i ethX radio off
with X = 1 (eth1 for 2.4 GHz) or X = 2 (eth2 for 5 GHz) [at least for 95% of Freshtomato routers]
Example:
wl -i eth2 radio off
Restart the wireless service, both 2.4 GHz and 5 Ghz radios:
service wireless restart