For a simple one way communucation where latency is unimportant, you might leverage any number of free services with apis you can abuse. You could send a tweer, or nastodon message, You could have one just update a file with a timestamp as a github gist or on pastebin or something.

There are also free mqtt broker services.

Or you could put one on the public internet with ddns name and talk directly to it.

Or you could set up a VPN between them - is logmein still a thing? - and then talk to each other directly via that interface

I think its allowed so long as you declare that its a saltire cross. Otherwise you might get banned.

I love elasticsearch, but man I really hate it too.