To be honest this isn't entirely T-mobiles fault. Your device which in this case is the iPhone is determine that the best connection is the US Cellular one over the LTE connection. The only thing you can do is ether choose to manually connect to the t-mobile one or to put the device in and out of airplane mode so that its forced to research for a connection. I know its a pain but thats the best solution for the short term.
Hello there wilderwire and thanks for coming to our support site!
I gotta tell ya, I agree with dragon1562 on this one. I believe the iPhone is designed this way in an effort to give you the most stable connection at that time. In some cases, it may be a roaming partner but it won't always be that way everywhere. Fortunately, T-Mobile is continuing to make network changes to expand our network footprint and provide even better service. As mentioned above, manually searching is gonna be the best way to latch on to T-Mobile based on the example you mentioned.