Freezer temperature alerts for a supermarket chain

Your supermarket chain has 400 stores with about 30 freezers each. Every freezer has a wireless sensor that reports its temperature once a minute. A small gateway in each store sends the store's readings to your cloud every minute, over the store's internet. When that internet is down, the gateway keeps each reading with the time it was taken. It sends them once it is back. A freezer that fails can spoil thousands of dollars of food in a few hours.

A freezer should stay at -18 °C or colder. Restocking opens the doors and can push it above -15 °C for up to 10 minutes, which is normal. The store's duty manager wants a text message when a freezer has been above -15 °C for 15 minutes in a row. The gateway also reports any sensor it has not heard from for 15 minutes. The manager wants a text for that too. They want one more when the gateway itself has sent nothing for 15 minutes. Food safety rules say every reading must be kept for 2 years.

How does a reading travel from the gateway to the manager's phone, and what decides when a text goes out?