Assuming you have an electrical source like chemical batteries, mechanical generators or even solar cells, With copper wire & iron you can have a telegraph. W

Assuming you have an electrical source like chemical batteries, mechanical generators or even solar cells, With copper wire & iron you can have a telegraph. With vacuum tube transceivers you get wireless. You just have to stockpile a lot more tubes than radios. Just in case you want to go big, telegraphs would require a huge pile of copper compared to what you could do with a single truckload of radios & tubes. If it does turn out to take a really long time for certain doom to finally arrive, rest assured it's proven this technology can reliably be built to last a century, and still operate even if it gets hammered by nuclear EMP. From a safe enough distance of course. If nothing else stockpile the tubes, everything else could be much more easily recovered from scratch.