2GB disappear in minutes

We had a 10GB day, 10GB night data contract since Aug 2018 and it was always more than enough, until the last 3 months. We are using it the same as before, but the data disappear. On Wednesday just after 8pm we check and had more than 2GB left. Just after 9pm we had 0 data left. Only 2 cell phones connected to our Wi-Fi router, no update or download done in this time. The last couple of months our router also can't get internet access for a hour or so and then suddenly access is restored without us doing anything.