There isn’t a simple solution. If there were, it would
get so crowded in that circumstance that any simple
solution probably won’t work as desired.
It is possible to have some sort of communications
with those locations, but not to the level of ease,
regularity, or reliability of cell phone connections in
normal times. Let me suggest some examples.
Standard short-wave voice contacts are possible when
the ionosphere is being cooperative, which may be at
different times of day and on different frequencies for
each target location. And that changes in various ways
with time, not all of which will be predictable. So it
isn’t as easy as just picking a convenient time and
frequency for everyone to meet, but, with high power
and a good antenna system on multiple frequencies,
You may be able to contact each station individually
on some sort of periodic basis. It will, however, take
some experience to learn what equipment is needed
and how to adapt to changing conditions.
The chance of making a contact (or the ability to do
so with lower power and/or smaller antennas) is
higher using Morse code, or some of the digital
(computer-to-computer) modes on short wave.
Morse code is slower, and for highest reliability with
weak signals and simple equipment you would need
to learn to decode it by ear to pick weak signals
out of the noise: another skill that requires training
and practice. Some digital modes can handle even
weaker signals, but aren’t all set up for passing
much more than an acknowledgement that you
heard the other station. Extended messages will
take longer to transmit.
There are automated systems (known as “ALE”) that
continuously check different frequencies to see if
contact is possible, that improve the chance of finding
an opportunity to get a message through. That adds
more complexity for improved reliability, but still won’t
guarantee that you can get through, or when. This
probably is most suitable for posting text messages
that get sent whenever conditions are suitable, so
not necessarily an immediate, interactive conversation.
There are other possible modes, including bouncing
signals off meteors, or even off the moon. The former
passes small bits of data at random times, the latter
provides a more predictable path (at the right time
of day), but requires more sophisticated equipment.
All of these, however, require experience to set up and
use to provide the desired communications. That
probably means a few years of practice and familiarization
to determine the required equipment and learn how to
adapt to changing conditions. To give a sense of
perspective, the ionosphere (responsible for shortwave
radio propagation) varies with the sunspot cycle (roughly
11 years or so), and ideally you’d want at least one
cycle minimum and maximum to see the expected
variation (although each cycle is different).
Based on observations in many circumstances over the
years, my philosophy is:
“If you don’t use it regularly, don’t count on it in an emergency.”
So using ham radio for your desired communications
isn’t something where you can just buy the radios and
store them in the closet until you need them. It would
be a long-term commitment to acquire the skills,
knowledge, and experience to make it work, along with
the required equipment and antennas.