Yes, I have the iPhone 7 Plus. It supports Band 12. And previously I had the 6S Plus which also supported band 12. It's not a frequency issue, it's their backhaul and saturation of towers. It's just unacceptable. I still have TMO (ATT is primary, and my second phone I use for international travel is TMO), so I turn it on often and hope and wish it's gotten better but it just hasn't. There are entire swaths of gates at SFO where you'll still see EDGE, and even a couple spots of GPRS or NO SERVICE. For an airport like SFO, that's a joke.
Downtown coverage is reasonable, until you go into a parking garage or low floors in a building. Maybe my expectations are high because I also have ATT and their coverage is far superior. I have friends who have TMO and say they're satisfied, and also have friends that loathe their horrid coverage\service.
What frustrates me about TMO is their coverage is there, but the network doesn't support it. For example, if you drive to Pacifica, there are areas around Skyline College where you'll get 3-4 bars, and generally it'll show as LTE, but more times 4G (HSPA). You'll have great reception, but if you try to place a call, use data, or send a message it just times out. This has happened all over the country for me across many devices on the TMO network, it's just unreliable. When it works great, it's amazing, but it's not reliable enough for me to count on. I know it's a second-tier provider, but it seems like they're putting their money into the wrong places.
Seeing the new plans, it seems like they realize their issues. They charged too little, which means not much capital to improve the network, they did unlimited, which now is unlimited but throttled streaming video and additional charge to have hotspot... so it's evident they self realize their issues they have and are working on it, but they have a long way to go before they're anywhere close to the user experience of ATT or Verizon. I travel 2-3 weeks out of the month domestic and international. I have had these issues with TMO nationwide, which is why I use ATT, which is why many use ATT. I have the TMO line solely to use for international when I'm out of the country. That's the one aspect where TMO obliterates all the competition.