If you've made the switch from AT&T to T-Mobile with your Windows Phone and are having trouble access T-Mobile's LTE Network take a look at your APN settings.
Windows Phone Central member MikeSo made the switch with his Nokia Lumia 900 and had trouble accessing the LTE network. He could see the network, just not connect to it.
Apparently the Lumia 900 wasn't set up for T-Mobile's LTE and the solution was to add an APN with the address fast.t-mobile.com in the Windows Phone Settings. After doing so LTE worked like a charm.
It's a fairly easy fix if you're Windows Phone is seeing T-Mobile's LTE network but won't connect. You can find the full forums discussion on the T-Mobile LTE settings here in the Windows Phone Central Forums.
Thanks, gp0777, for the tip!