Yes. You need to extend your rental even if you will not use the internet on the last day.
Why an extension is required
WiFiBOX is a rental service.
Rental fees are based on how long you keep the device, regardless of whether you actually use the internet.
If your last day is a travel day
Even if you will only be traveling and do not plan to use WiFi on your last day, you must extend your rental if you will return the device after your rental period ends.
Whether an extension is required depends on your return date—not on whether you use the internet.