Ask HN: Internet without ISP's

Wild hypothetical I was pondering the other day: Suppose that there were an apocalypse scenario and society totally breaks down, meaning that there are no longer any ISP's out there that are functioning. Also suppose that someone hears rumors that there is a a group out there re-establishing the internet. Everyone knows you are a hacker so they turn to you and ask you if you can find and connect to it. My question is, how would you go about doing that without any functional ISP? Ive spent some time reading online about BGP's such as mentioned here:

https://superuser.com/questions/900021/how-to-internet-without-isp

Anyone have a more comprehensive idea on how to think about this? Anything that can get me started playing around with these ideas now? I feel like as a mental exercise being prepared for this scenario would teach you a tremendous amount about the finer details of networking (of which I am not an expert). I would hate to let everyone down, if that scenario were to arise. :)