There are many cellular coverage maps out there that you can overlay with population graphs, but I can't link you to anything specific besides just a coverage map. But this isn't a problem unique to India. Many countries have holes in their coverage, including the US. There are many parts of the US where you'd be lucky to get 1 Mbps down on 3G. Or where your only broadband option is DSL at a few Mbps.
https://www.pcmag.com/news/native-american-tribe-gets-early-...