there has existed for somehing like 35 years an analogue modem for 27MHZ, 142MHZ licenced and unlicensed. It's just a little PCB you solder to your mike / speaker points. There are multiple private 24/7 uplinks around the world. so you can mail and surf as you would normally, but slow. Around 2K baud, So try to sync your block chain on that.

As a post apocalyptic internet it's fine.