Interesting topic.
Recent read about how Iran hid it’s nuclear program from Mossad, USA, everybody.
NO SIGNALS of any kind. Couriers and hand written paper.
Works perfectly if you have very limited points to communicate with.
I’m in the “no signals or electronics” camp.
The previously mentioned “one time pad” is thought to be unbreakable, if everyone does their part.
Takes some time and planning to set up.
en.wikipedia.org
I would use shortwave radio as a “numbers station”, everyone on frequency on the planet can hear the numbers, but only the people w/ the decode can read the message.
en.wikipedia.org
Here is another idea - old time FAX machines, on an old time POTS (Plain Old Telephone Service) analog dial tone line. Not that it is secure at all, but the technology is so ancient and arcane, I’ll bet no one has tech deployed to record & decode it any more.