I've had lots of luck in 5 or so trips, all a different distances down island of just walking to the top of the tallest dune. It's not always the greatest connection, but I've been able to check weather/tides and send a text. We camped last year about 20 miles down, set a phone and a bluetooth speaker at the top of the dunes, and listened to the UT/ISU game at our campsite.
This also worked last year all the way at the cut, when a storm was blowing in. Now I will admit, I have no experience between MM20 and the cut, because I have never needed to stop between those points and check.
All this to say, I do have a buddy that has one of those Cell booster antennas that he runs up a telescoping flag pole on his little airstream. He works from home, so he travels and works for weeks at a time, and this was his solution for 4 weeks in BiBe earlier this year.
Edit to add: my buddy referenced above has the Weboost brand Imma linked above.