A lot of ideas here. See the overview in Kris De Decker, How to Build a Low Tech Internet (although they do not mean low tech so much as cheap/easy tech).
Connects with sneakernets via drive by netowrking and data mules.
Smartphone mesh networks
Others off-the-shelf mesh systems must exist, but the only ones I am aware of are special-purpose networks built for one-off projects.
Packet Radio (NPR), which I chose to publish under my call sign, F4HDK, as a nom de plume. It supports today’s de facto universal standard of communication—the Internet’s IPv4—and allows data to be transmitted at up to 500 kilobits per second on the popular 70-centimeter UHF ham radio band. […] it is still faster than the 1.2 kb/s normally used by AX.25 links, and the 70-cm band permits long-distance links even when obstructions prevent line-of-sight transmissions.
Legality of accessing spectrum
I am not an expert in the legality of these things. Research before trying.
Sending data over human audible frequencies is slow and fraught, but possible.
minimodem is a linux classic here AFAICT.