Check this out. Supposedly if you have an unlocked IPhone 5 that supports AT&T/TMobiles LTE band and are jailbroken you can enable TMobile LTE right now if you live in the coverage area:
http://leimobile.com/how-to-enable-lte-on-iphone-5-for-t-mobile/
So you purchased an unlocked iPhone 5 before T-Mobile released their new version of the iPhone 5 that supports 1700mhz. This could be the AT&T model or the officially unlocked version sold by Apple. Please understand that if you have a Verizon iPhone 5 you will not be able to connect to T-Mobiles LTE network. It does not have the correct LTE band. Currently T-Mobile only has a few cities listed that have support for LTE and they are upgrading towers all year long.
Now that you have an unlocked AT&T model A1428 or an officially unlocked model A1428 you can proceed to follow the instructions below to load a custom carrier file onto your iPhone and enable LTE. This will only work on A1428 model iPhone 5′s.
This is the guide to enabling LTE on your iPhone 5 model A1428 for T-Mobile.
1. Open Cydia
2. Tap Manage and then tap Sources
3. Tap Edit then tap Add and put the following URL into the text box:
http://v.backspace.jp/repo
4. Once the URL is entered in tap Add Source and allow for it to download all of the repos info and packages.
5. After your iPhone has refreshed, tap the v.backspace.jp/repo field or go to the search bar and find the package CommCenter* patch
6. Once you have found it, install it and then reboot your iPhone once completed.
7. After the iPhone has restarted, open Cydia once again.
8. Search for iFile. The free version includes basic features. You will want to purchase it to take advantage of all of the features.
9. Once you have found it, install it. No reboot is required, though you can do it just to be on the safe side.
10. Once iFile has been installed, close Cydia and click on this file from your iPhone and click Open in iFile
11. Once youre in iFile tap Unarchiver and 3 files will show up above T-Mobile LTE.zip
12. At the top, tap editand tap the dots with a check mark next to carrier.plist, overrides_N41_N42.plist, and overrides_N41_N42.pri
13. Tap the clipboard icon at the bottom and tap cut
14. Now tap done at the top and tap the house icon at the bottom then tap Library, then tap Carrier Bundle.bundle (the one in blue), and then tap edit at the top again and tap the clipboard icon. Tap paste and it will auto inject the carrier files into the carrier folder overwriting the other files.
15. After the files have been placed, exit iFile and restart iPhone. After the iPhone restarts, allow it to acquire signal and display the T-Mobile logo then go to where you edit the APN settings (Settings >General>Cellular) Once that is done, enable the LTE toggle and youre good to go.