Each voucher code is linked to a Wi-Fi plan. E.g. A 3 month Voucher code is for a 3 MONTHS plan, a 1 DAY voucher code would be for a 1 DAY plan.
When entering the voucher code the correct Wi-Fi plan must be selected. The plan that is highlighted by default is the 1 DAY plan. This must be changed to match the plan that voucher applies to.