I stopped and got a SIM at AT&T to put in the second SIM slot. He provisioned it with the IME number from the 950XL
Got home and that SIM will only connect at 4G
The SIM in slot 1 is out of my 1520 and it an AT&T 1520. In the 950XL it connects at LTE
After reading this thread, I took the SIM out of my ATT 640 and cut it down to fit SIM2 in the 950XL
It too connects at LTE
Put the 4G SIM (new one from today) in the 640, and get 4G
So, I am convinced you can NOT get LTE if AT&T provisions the SIM with IME numbers from the 950XL
Having said that, I am betting that AT&T and Microsoft WILL work this out shortly. They BOTH have a vested interest in this specific case.
But I won't wait. I'll simply go to AT&T store and have them provision a new SIM for the 640 using that IME number and I will have 3 LTE SIMs. 2 for the 950XL, one for the 640
If you are wondering why I have 2 SIMs in the 950XL, my employer provided SIM is a 5 GIG data plan, and my home Family plan is 40 GIG rollover.
So, now with this AMAZING phone I get to switch back and forth, have both texting accounts on one phone, and HOTSPOT for my Surface Pro 4 with either the 950XL or throwdown 640.
Oh, and this phone does everything my beloved 1520 does better. It might not be as sexy, but it is a powerhouse