What are possible fallback-communication channels besides the internet?

The easy way is to buy a satellite modem. It’s not cheap, but isn’t challenging to setup.

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.

Phone, Ham Radio, BBS (via phone), Bulletin Board (the wooden type). During the fires in our area a few stores had a makeshift situation update board outside of a few of the bigger stores.

There are compact, off-the-shelf products, such as cellular and dual WAN routers, that can switch automatically to a second Internet connection, which could be delivered on a different medium by a different service provider. For example, at a site with a fiber optic connection, you might also be able to get cable modem, DSL, fixed wireless, or cellular service, though there is still a risk that an upstream failure—such as a fiber cut on a long-haul route—could affect multiple providers.

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?

Ham radio for one.

AX25