Update: Three different T-Mobile employees lied to me ("Any tablet that says 4G or LTE works"). After I got the tablet, I tried to use it with my sim card. It wouldn't work, so I called their support. They suddenly mention something about an IMEI number. I read the number off the device. They now tell me it won't work.
Now I have a cheap, low-powered Windows 10 tablet wasting away in my desk at work.
I suspect you can't get just any device you want with an always on connection, and I further suspect I'll have to get a "hotspot" if I want to use my data plan on a linux device.
I asked this question here because I felt it was very difficult to find exactly what I was looking for.
I was right.
I used Google. I searched Amazon, ebay, newegg, T-mobile (my wireless provider), and even Google's shopping. Then I found a site called Frequency Check that would let me search by frequency.
Having been given 1700/2100 bands by a guy at T-Mobile, that's what I primarily searched for - but later I was told by multiple T-Mobile sources that anything with LTE or 4G would work.
Anyways, the models I was finding on Frequency Check were not showing up in my searches. It turned out that, when I could find them, they were only available on the foreign versions of sites. Out of desperation, I started to consider trying to make a purchase from a Chinese site using Google Translate.
The Holy Grail
After much searching and wondering what I was missing (since I knew the market for this could not have completely died in the USA), I finally decided to watch some CES videos on Youtube, and then I Googled "CES tablet" for a rundown, and finally found what seems to be an ideal 2 in 1 tablet, the Alcatel Plus 10.
This still seemed only available from non-US sources, at least directly from Alcatel's site. But then I found a new in box factory unlocked one on ebay.
Here's my original requirements lined up with my conclusion:
"T Mobile's data network (1700 and 2100 band?)" - The keyboard has LTE, and by some descriptions can provide connections for up to 15 other people.
"easily rooted" - well, it comes with Windows 10 home, and it is unlocked, so while I won't get the experience of rooting an android tablet... I only have so much time... and I'll have administrator access on the device.
"can run Linux or Android's linux" - Windows 10 home should provide me with a Linux subsystem, but I'm also entirely prepared to install straight Linux on it.
"Bluetooth" - this was important because I wanted to get a clamshell keyboard, but I suppose I can do without it. However, I just checked the specs and it's supposed to be included.
"a clamshell bluetooth keyboard accessory would be good, mouse even better" - well, I suppose I can get a bluetooth mouse now
"use it like a regular tablet to navigate when I get off the subway" - I do worry about this a bit, it's a little larger than my almost 8 inch tablet that I used to use to navigate, and google maps was an app only a few taps away, but the keyboard is supposed to include GPS - so I guess we'll see what happens.
"use it like a laptop at meetups" based on the form-factor, I think I can stop lugging my larger Lenovo Yoga around in a backpack now, a major improvement for my lifestyle.
"pipe video to a big screen from it when presenting or demoing code" - It has HDMI mini, so I suppose I need a cable and/or an adapter.
"utilize it as a mini screen to expand my laptop's desktop size when working at home" - Still not sure how I'll do this, but I still want to.
The hard part about this specific model is that I couldn't easily find a seller in the US.
But I scored - I managed to snag what was advertised as the last one on ebay, new in the box, unlocked, for ~$310 including a 2 year warranty (which I hope not to use, and didn't mind spending less than $30 for).
As I was prepared to spend as much just on a tablet and more for a clamshell laptop-y case (probably another $100) I think I have found a solution that doesn't cost an arm and a leg (another of my requests).
A possible competitor was the Asus Zenpad 10, also ~$300, with Android and an optional dockable keyboard (noted in the early promotional material) but I could not find the optional keyboard accessory for sale anywhere- which I would expect to be pricey anyways.
I am usually reticent to buy things as complex as computers or phones on ebay, but given the difficulty of my search and my inability to find a seller through other venues, I really had no choice.