Thank you for reaching out! I was able to look into your account to make sure everything looked okay, and it seems that everything on our end looks alright. 50GB were consumed before 9/22, when the account exhausted it's monthly allotment. It appears that about 16.2GB were used on the 16th, so it seems the data meter refreshes after the first full day of usage.
"The next day was even worse - 22 Gb used. "
Was this use in one day? You may want to consider installing the free network monitor Glasswire on any compatible device.
Glasswire will only report usage from the devices it's installed in. There are routers that monitor overall network usage and this is something you may consider getting. I don't know of a specific one, but someone else might.
Glad you figured out where the data drain came from.