According to T-Mobile MVNO Ultra Mobile, iOS 8 fixes the bug and Ultra iPhone users who upgrade to the latest iOS are finally able to use LTE. After upgrading to iOS 8, you just need update your iPhone's cellular data settings as follows:
- Tap the Settings icon then tap Cellular then Cellular Data Network
- Scroll down to Cellular Data and tap and set APN: to wholesale
- Scroll to MMS and tap and set the following:
- APN: wholesale
- MMSC: http://wholesale.mmsmvno.com/mms/wapenc
- MMS Proxy: 22.214.171.124 :8080
- Tap Cellular
- Press the Home button
- Power the iPhone off by holding down the power button
Turn the iPhone back on and you should have working LTE. In addition to Ultra Mobile, the above settings should also work with PTel and Giv Mobile. For other T-Mobile MVNOs, check your operator's web page or call them for their recommended iOS data settings.
I don't have an iPhone to test this with but if you do please let us all know if it worked or not.
Source, image: Ultra Mobile on Facebook