How long does it take to train a cashier?

Minutes, not days. A cashier taps their name, types four numbers, taps dishes, presses how the customer paid and hands over the receipt. The things that need thought, such as giving money back or cancelling a sale that already printed, need a manager anyway. Give somebody one quiet hour on a real device before service and they will be fine. Staff, PINs and hours in the docs lists what each role may do.