In times of war, internet service is likely to be unreliable. Are there any messengers that support a mesh-based store-and-forward model, that do not require an internet connection?
I think it's also worth thinking outside of the digital box. Radio - CB/ham/amateur radio in particular - requires no infrastructure and is far more expensive to block than the internet.
Very true in peace time, but it's in clear and easily traceable which makes it dangerous in war time; also there's little development in encryption over HAM/CB since it's illegal pretty much everywhere; encryption devices can be added externally, although for the above reason they're not immediately available at the next radio shop. Moreover, nowadays military forces can pinpoint immediately the source of a rogue transmission by measuring the phase delay of a signal on multiple fixed antennas, that is, they don't have to waste time while pointing directional antennas around to find the direction; therefore you push the PTT and after a fraction of second they have your location on the map. Now I'm not sure the Russians have this infrastructure in place in Ukraine, but they could if they wanted.