I'm having this issue on a Sprint iPhone 6 16gb.
My phone is only picking up an LTE signal about 50% of the time. I have to constantly switch to Airplane Mode and back to get it to find one. It wouldn't bother me so much, except that 3G doesn't work anymore. Siri can't respond anymore on 3G, Apple Music (not that I plan to use it after the free trial) and Spotify both refuse to play on 3G as well. Websites load very slowly. Calls still go through fine, so at least I've got that going for me.
As I've said elsewhere, this is my last iPhone, and I've owned every iteration since the 4. This amateur nonsense with the software is unacceptable from the world's largest consumer electronics company. What good are frequent updates to the OS when they ALWAYS break crucial functions? I'll stick with my Mac for music production/photography, but iOS is no longer for me.