+2 I agreeYou write it on a piece of paper with a burnt stick. Fold it and send with a runner.
Definition of SHTF.
Advance plan with the family during a SHTF event. Don't plan on relying on any type of current communications. A federal comm officer gave a class about comm during Kitrina. The found out the old CB radios were the only thing they could rely on. Of course when the SHTF, the radio frequenies will be jammed. Rely on yourself and you advance plans.
What about a ham radio rig that only does cw (Morse code). Can travel great distances on little power.
And is not the most difficult thing to learn.
https://morse.withgoogle.com/learn/
I was going to reply suggesting morse code also thanks for the link.