How to setup expiring password for Wifi on MTNL modem

[Fozia]

My friend has a small shop for food items. To attract his customers he is planing to give them free wifi. So he bought MTNL 1mbps plan. He is using the default MTNL router that comes with the connection. Now he wants a option through which we can setup expiring password to the wifi. Like a customer can use the wifi until he or she is in the cafe. And after a hour the password must get expired on its own and a new password must be applied. How can I find this settings in the router. Like I can add around 10 expiring passwords and share.