https://cellmapper.net (I don't particularly like it because the community vs. commercial aspect remains unclear. But they had data in all places I have looked.)
CellMapper doesn't seem to work on iOS. At best, it shows tiny triangles that might be a few cell tower sites, but I know it's missing a bunch of towers. And it doesn't show me signal strength or performance maps -- either as a weather style isocline, or with hex style grids.
It shows only the data its users have gathered. Here in Finland there seem to be many actice users who confirm tower locations. E.g. in the US there are many calculated, unconfirmed tower locations. Obviously not as many technically inclined volunteers.