If you're struggling with cell coverage, there's something you should know: your phone matters.
In the FCC’s guide to understanding wireless coverage, they discuss topography (the surroundings), capacity (the number of people on a network) and network architecture (the location of cellular towers), but fail to mention phones. However, there are several device-related factors that can play into your coverage strength.
It used to be that cell phones were manufactured so that the antennas were on the outside. Watch any 90s flick for evidence of this. Nowadays, they're inside; this is great for cosmetics, less so for coverage.
Sometimes these interior antennas can be impeded by your cell phone's case. This is why you might notice better coverage when your phone is "naked." Phone cases are probably a worthwhile tradeoff for the accident prone (myself included), but perhaps less so for the truly coverage-challenged.
Simply put, newer phones get far better coverage than older models. This is because they have the radio technology to tap into newer, faster "spectrums" rolled out by carriers.
For example, last year, American carrier T-Mobile introduced a spectrum known as Band 12 that it says works four times faster than its predecessor inside buildings. The iPhone 5S doesn't have a radio that works on Band 12, whereas the iPhone 6S and 7 both do. Same goes for other carriers and phones: the newest models will be more apt to have the built-in technology to operate on the latest spectrums.
Make sure that the software and firmware on your phone is up to date—both can affect coverage, for the same reasons discussed above. While the latest hardware helps you connect to certain frequencies, software guides the hardware and how it performs. The latest software might better fine-tune the specific frequencies that your phone looks for.