The universal pattern: all major carriers perform well in cities and established tourist towns, and the differences emerge exactly where you're describing, mountains, islands, national parks, and stretches of highway between towns. In most countries one network has invested most heavily in rural masts and is the acknowledged coverage leader outside cities; locals, guides, and drivers always know which one it is, and asking them is faster than any research.
For data rather than folklore, use crowd-sourced coverage map websites and apps, which aggregate real users' signal measurements and show honestly where each network works, unlike carriers' own optimistic maps. Check the specific beaches, islands, or trekking regions on your itinerary rather than the country average.
A few structural truths help set expectations: islands with resorts usually have decent coverage while uninhabited or barely developed ones don't; mountain valleys can be dead zones even in well-covered countries; and popular viewpoints may have signal while the trail to them doesn't.