ONTs don't in general pick up an IP address, so there is no "lease" for them. They are managed via separate management frames from the OLT.

But switching off for a minute or two isn't a bad idea anyway, because allows capacitors to fully discharge. Then you know it's definitely starting from a clean boot.

in addition to what @candlerb has said, you may only see an ont taking an ip at enterprise level, and some sfp pon trancievers. Otherwise its a level 2 device and is just acting as a media converter with a ethernet presentation at the end.

As an user you can not flush a pon connection, as its a shared connection. OR can do that. If you need to clear a stale session with your service provider .. disconnect the pppoe connection and and reconnect it . You can do this via the router's interface or reboot the router and you also can do it via power cycling the ont or router.

poert cycling the ont is a last resort type thing.