Assuming you have nailed down problems like wireless antenna connections and so forth, the typical reason for bad performance is that the network interface has a crappy open source driver, which is acting up and causing issues.
The way you have written you post, I cannot tell if you are having problem with the wireless or the wired network, but you may want to look up the exact chip online, which may help you find a solution. Often enough all you have to do is disable the feature that is creating problems.
I've got to say that I've been impressed with the upper-tier of T-Mobile support with some of the challenges we've faced. Thanks for posting back, as getting back-up transport over LTE is on my list for this summer.