I have been using cell phone data full timing for the last 10 years. At first it was real spotty. At one time I split my wifes phone plan and mine, Verizon and AT&T because there was a huge difference in coverage. Keep in mind, we travel from one end of the country to the other pretty constantly, so cover a huge area.
All of the cell phone coverages that I experienced, or talked with others who have T-Mobile, Sprint, Cricket have regional strengths and weaknesses. In my experience Verizon has worked the most often, sometimes not as good as the other predominant services for that region, but still works.
There are some new gadgets out there that I have not explored. They tend to take a pretty good up front investment. But if it is dependent on the use of a cell tower, you are going to get into places where it will not work, fewer these days than even 5 years ago, but they still exist.
So my Verizon MiFi Hot spot runs me $31 per month for 100gb of data. It is called an unlimited plan. But after you use 100gb of data it is limited, they all it throttling, it becomes so slow it's unusable. It has worked in every place I have been this year, and in some places it's really fast. Keep in mind I have sold my soul to the company store, I have 2 apples watches, 2 i-phones, an I-pad, and the Mifi device. Add the insurance policy and my bill runs about $300 a month.
One note when you do your research, and someone is telling you that there service is great, ask them where all they have traveled with there service.