The 6 Plus doesn't support Band 12 (700 A Block Spectrum) aka low band. With low band spectrum the signal carriers farther and penetrates objects (buildings, trees & etc) better even though it is a bit slower. Until a few years ago AT&T and Verizon used their size (duopoly) and bought up 93% of all the available low band spectrum when it went up for auction so all the other carriers essentially didn't get any. Now to prevent that at the most recent auction the FCC put protections in place so that couldn't happen again. Anyway a few years ago Verizon which had built its entire LTE network on its low band so they had coverage everywhere was coming to a grinding halt because as I mentioned above it is slower and doesn't have the same bandwidth as the higher bands. So Verizon came to T-Mobile and offered to sell them low band in a lot of markets for cash and some higher band spectrum to which Verizon called it XLTE and of course T-Mobile calls their low band extended LTE. Even though this was known when the iPhone 6 came out and the iPhone 6 has 30 LTE bands built into it, for some reason Apple decided not to include Band 12 even though Samsung and others had started too or at least included a Band 12 capable radio so they could update the firmware and activate it. Apple being Apple decided not to do either.
Now looking at an unofficial map but which tends to be quite accurate T-Mobile did get Band 12 spectrum in the Owensboro area and it is available now for deployment. (Some markets had to wait due to channel 51 issues) but doesn't look like it is live yet but does look like it is getting close since if you drill down on T-Mobile's map and click on some areas close it shows pockets of Extended LTE so it may be picking up a tower a ways a way but again means the tower upgrades are getting closer. To give you an idea of the propagation of signal from a tower it generally takes 4 higher band towers to cover the same are as 1 low band tower, so that's why you are seeing better signal closer to towns since that is generally where the towers are because it covers more people but also a tower requires what is called a backhaul which is the fiber data line which tend not to be available away from populated areas which is why low band is so valuable.
Now with that being said first the 6s iPhones and above do support Band 12 so as they continue to upgrade towers things will only get better however and this is anecdotal and my personal experience and that is iPhones antennas tend to seem to struggle for signal more. If you search signal issues on this forum you will see iPhone complaints far outweigh the Android complaints. Second is I have a friend who complains about signal at his house (he loves T-Mobile for all the other perks so he keeps them) and didn't believe me when I told him it was his phone not T-Mobile because I know there is a tower fairly close to his house and he has a Band 12 capable iPhone yet my signal is always substantially stronger than his standing right next to him in his driveway.
When looking at T-Mobile's coverage map make sure to drill down, when I pulled up Owensville it looked really good except around Fordsville but when drilling down the magenta got lighter and said "Fair" in a lot of areas outside the cities. Fair can be spotty especially when indoors or in heavily wooded areas. Now of course Band 12 will eliminate a lot of that.
Here is a link to that 700mhz T-Mobile map, again not official T-Mobile but quite accurate and has good info on it.
Thanks for the very lengthy and detailed reply! Very informative, just the way I like it and learned a good bit from it. Well the next question is, Lets say im traveling to Calhoun, KY (pictured below), about half way, I loose signal all together (coming from Owensboro) and I will not have any signal at all for texting, calling or data until I reach the outskirts of Central City, KY (not shown in the map due to the 4G LTE box showing up, but its right close to Bremen, KY which you can partially see with the box covering it) Even though this map shows I should have Voice and Textinv capabilities, why would I not be able to take advantage of that even though it shows no service? I use to be on Tmobile Prepaid service a few years ago but had to get away due to the limited coverage which this map use to be mostly all white (except for Owensboro which was the only limited area covered) until recently back in December when they updated this with this beloved 4G Pink coverage almost everywhere now in most of the rural areas where I travel.
I have studied band 12 moderately over the past few months but your explanation of it makes it more simple to understand, but being in a moving vehicle shouldn't make much of a difference as it does not have much to penetrate through unless of course im stationary in a structure such as a home residence or a business in a rural area/small city.
I have been thinking of switching over but with coverage problem and her iPhone, I am very hesitant at the moment unless it is a iPhone problem with coverage, then I will try it out on a Android device as I prefer androids over anything else and its sufficient enough for what I do. I have been looking at a LG G5 (H830 Tmobile Brand) model to use and with it having Band 12, hopefully it does wonderful and it has coverage in all the areas where she just simply "looses service"
If you notice on the box it notes "requires a cable device" which is Band 12. If you originally select my phone is not on the list or another carrier you will see the map is not quite as filled in especially between Owensboro and Calhoun there is a lot of white spaces also when you zoom in closer you will see it is actually a lot of "pink" rather than magenta once you get away from the cities. From what I see from the 700 map I included it looks like there are some sporadic sightings of Band 12 in Louisville and Princeton which is a good sign that they are starting to add Band 12 radios to the towers in the area. I know when they were adding Band 12 out where I live I would see it then it would go away then be back more constantly until they announced the market had been done. Now one thing to know is that Band 12 is their secondary frequency and in most markets is a 5x5 channel instead of their normal Band 4 (band 2 in a few markets) primary frequency which depending on the market run from 10x10 to 20x20 wide channels so all Band 12 devices will only use Band 12 when Band 2/4 is unavailable.
Remember even is driving that the signal still has to go through trees between the vehicle and the tower. I know at my old house which was more out in the woods I could see the tower in the fall/winter but during spring and summer with all the leaves on would lose a bar or two because there were a lot of trees between the house and tower about 2 miles away.