The usual business model is to make people pay for more data than they use (since data both “expires” at the end of the month and you can’t usually add more if you run out), along with contracts where people keep paying for non-rendered services because stopping payment would damage their credit score (which means networks can oversell beyond their capacity to actually provide the service they sold). Also, being a host network themselves means their network costs are near-zero, where as Zevvle has to pay a premium to their host network by virtue of being an MVNO.
Zevvle’s problem is that they are both too small to get enough of the “leftover data at the end of the month” to make a profit, and are too “nice” in the sense that they give you better tools to monitor & control your usage (Z allows you to change your plan every month where as most contract plans don’t) as well as being able to share the data pool across multiple SIMs which makes people have even less leftover data - ie. they are using what they’re paying for.
Presumably, even with the above, given how lean Zevvle is compared to the big players (cf. the advertising campaigns you mentioned), it could probably still work if they manage to get a sizeable customer base… but you’d first need to get there somehow. Oh and obviously there’s nothing preventing their host network from suddenly jacking up prices to cash in and Z would have no choice but to pay.
Unfortunately in a world with unfair competition and a regulator asleep at the wheel such a fair and sensible business model simply can’t win. I wish the concept of paying for “data” that magically expires at an arbitrary date and whose fungibility is arbitrarily restricted would become outlawed, but too many people make a profit off this model to ever see that change in our lifetimes.
