I have been with T-mobile since I moved here 17 years ago. They were PowerTel back then.
First, Elevators are big metal boxes that make reasonably effective Faraday cages. Simply put, they block Electro-Magnetic waves such as cell phone signals. Your experience is typical of all mobile phone users in elevators.
Atlanta is a T-Mobile town. They have a very heavy local presence business-wise as well as a high density of towers. I worked in Canton a few years ago for about six months and had no trouble getting a signal.
I like the fact that T-Mobile de-coupled the phone from the line, but still allows you to finance the phone at zero interest. Right now, they have a Father's day special where you buy a Galaxy S8 and get one free if you open or transfer a line to them. T-Mobile and ATT are signal compatible so you can unlock you ATT phone and bring it with you.