Crossing the Atlantic with radio is totally doable, even with low power if you accept communicating at a few bits per second (see for example https://en.wikipedia.org/wiki/FT8 and https://wsjt.sourceforge.io/wsjtx.html). As it's time-of-day dependent you'd need to have a way to switch bands (daily propagation patterns on 7 MHz are totally different from 14 MHz for example) depending on the ionospheric conditions, and a multi-band antenna. I'd rather use a pair of beams (this kind of thing: https://en.wikipedia.org/wiki/Yagi%E2%80%93Uda_antenna#/medi...) pointing to each other rather than a dipole, gain is much higher so you need less power. But even using multiple bands, there will be moments where no connection is possible at all.

There's also a satellite... https://en.wikipedia.org/wiki/Es%27hail_2, but of course it would be a single point of failure.