Not just a hypothetical — during the December bush fires in Eastern Australia some towns were left without telecommunications when cell towers and power lines were burnt down. People ended up relying on AM radio for fire and road info, and HAMs set up temporary radio systems until things were restored.

e.g. https://www.abc.net.au/news/2020-02-01/amateur-radio-skills-...

Atlassian founder Mike Cannon-Brookes helped get solar power set up for the cell towers that were left without power:

https://www.sbs.com.au/news/the-feed/atlassian-co-founder-ro...

So your best bet is to have a working battery-powered AM/FM radio and a solar panel to recharge the batteries.

There are plenty of half-finished mesh projects that don't have critical mass. The biggest problem with any kind of disaster preparation is that it won't work when you need it unless you use it on a regular basis, so ideally you'd want a mesh that's better than regular ISPs. Nobody has been able to crack that.

You should look at APRS[1] if you're interested in ham radio:. It's designed for message routing and location data.

[1] http://www.aprs.org/

What's your definition of a long time? And how big of a geographic area?

Multiple satellite network projects are underway.