I do not know whether there are compact solutions that allow you to fallback to SMS / phone line / radio waves once the internet connection dies for any reason.
Really fascinating to me
Beyond that, there are variety of ways to do things, depending on what you want to do. I worked at a place 20 years ago that used a laser system to connect two buildings. Another used a microwave system.
An old friend who’s a farmer uses modified antennas and cheap routers to link various barns or other locations on the farm.
Installations that have to stay connected even during widespread network outages may use technologies such as private microwave radio, satellite (Iridium phones or VSAT), or HF radio with automatic link establishment.
The right answer depends on the requirements, of course. What are the capital and operating budgets? What throughput is needed? Is it a closed network of nodes designated in advance, or do you need to reach the Internet or PSTN? Are you concerned only with failures due to malfunction or accident, or also intentional interference, and if so, by whom?