I am planning on ordering a t-mobile S5 for AT&T as well, t-mobile has zero coverage in my most common adventures in Utah backpacking/camping,etc.
With an unlocked simtray, does at least H+ 4G work? I can go without LTE coverage until I device to hassle with them like you did to provision the sim properly.
I eventually got mine provisioned correctly after giving them the numbers from another phone for AT&T.
Hope this is clear as mud now lol!!!