TMO is pretty amazing across DFW.... Very few weak or dead spots.. but you are correct.... get outside the Metroplex (and I think saying 20 miles out is being generous) and you are pretty much dead for anything remotely close to a data signal.... I know when I go up to Winstar or Choctaw T-Mobile is mostly useless all along the major highway (and the same thing on the drive down to San Antonio) if you live, work and play inside a decently populated area T-Mobile is pretty good... a couple of pockets along the highway you'll get a good LTE signal but those areas in between are horrendous....
but I spend 99.9% of my time in Coppell, Irving, Grapevine, Lewisville and occasionally Arlington when the kids want to go to Six Flags... I also use that $30 Walmart plan with unlimited Data and Text since I rarely use more than 100 minutes per month... So you can't beat that.... The last time I drove back to TN I just got an AT&T prepaid SIM and card for one months use.... I'd pop that in for the drive and then swap back out my T-Mobile SIM once I got to the mother in laws house in Nashville......
EDIT: Switching out for the AT&T SIM isn't ideal if you travel often.. Which I use to do.. but if you take a trip or two a year it's at least an option if you are out of contract and have an unlocked device..