What happens when a day pass’s daily data runs out?

Nothing stops working. A day pass gives you a set full-speed allowance each day; once that is used, the connection continues at a reduced speed until the next day resets it. Every day-pass plan prints its own reduced-speed figure on its own page.

Why there is no single figure on this page

The reduced speed is the operator’s own figure, and it is not the same on every plan. Averaging it would produce a number that is wrong for most of them, so it is printed where it belongs: on each plan’s page, and on a destination’s page wherever that destination’s plans agree closely enough to state it in one sentence.

Working as described is not a fault

A day pass slowing down after its daily allowance is the plan doing what its product page says it does, so it is not something the refund policy treats as broken. The Refunds page keeps “I changed my mind” and “it does not work” apart on purpose, because the two have different rules and different rights.

Spotting a day pass before you buy

Every destination page shelves them apart from the trip-length plans and prints the same note above them:

These are priced per day — one pass covers a single day. Each day brings a fresh full-speed allowance; once it is used, the speed is reduced until the next day.

Day passes are shelved apart from trip-length plans on every destination page for exactly that reason: one is priced by the day and the other by the trip, and a single price-sorted list of both makes the cheapest day look like the cheapest holiday.