If you can get a few hundred watts of electrical power, StarLink is an option for broadband connectivity via satellite when all the local communications are down. Don't know why, but Star Link reminds me of Sky Net.

This is going to be controversial but...

Linux is not really suited for the post-apocalitic no-internet world, the way the repositories are built and software is packed (almost nothing is static, a lot of dependencies on other packages everywhere) just makes it really impractical and hard to deal with those scenarios. Flatpak / containers and friends even make this situation worse because you can't easily mirror the repositories and there's no straightforward way of exporting a Flatpak as a solid file that can be shared around and installed everywhere - the current tool for that doesn't account architectures and dependencies very well.

Windows however is a much more solid and good option, yes, it's painful to hear this but in Windows you can get an exe from a friend in a flash drive and it runs as is. Same goes for installers, reinstalling the OS etc. There's only a couple of .net framework installers that will cover dependencies for 99.99% of stuff in a few MB. The same goes for macOS, however it depends on a lot of software signing nowadays and certificates that can expire and you then have a problem.

A piece of software always runs locally. It is in some cases those who needs to communicate with the server fail to deliver the usual function you expect when offline.

Please do not confuse one to another.

And perhaps you can start by complaining which services you are using heavily rely on the server side? General questions attract general answers and IMHO you are better off just search on the internet.