Earlier quoted context omitted.
I cannot imagine how that would work when there are gaps between populations, such as villages. There are so many places where you have gap of several kilometers until the next village or city. How do you plan to bridge that gap? And if someone tries and fails to send a message across such a gap, is it stored on every phone in the vicinity? That could lead to unwanted conditions (large queues, multiple delivery), whi…
There's going to be people who travel more often between the two network islands already so there's several ways you could do it. The network as a whole could track nodes who often see rarely seen nodes and navigate packets towards those 'bridge/traveller' nodes or the nodes themselves could keep track of nodes they commonly see and choose to cache more messages intended for those nodes it thinks it might reach in vi…
One option --but it might require some centralization-- is that people announce they're going to travel, and stash a bunch of messages. If the sender can provide information about the geographical destination, that could help.
About monetization: remember what money did to the internet. But the above option would practically invite some form of payment. Thinking of Perth, that would lead to a kind of "Mad Max meets Johnny Mnemnonic".