I got this phone at the end of November and it started giving me problems with the mobile data a week or 2 before the update to Kit Kat. When my reception is at 3 bars or less, using apps that require online connectivity, sending or receiving mms, browsing the internet, etc it intermittently hangs and when I go to settings, the mobile data displays as disconnected. Restarting the phone kicks it back on, but it will do it again shortly after.
Calling and texting works fine though.
I've factory reset the phone multiple times, changed the preferred network to cdma/lte, and added an APN all with no luck. I've never rooted it or installed any roms but I have a feeling that if I do try that, it won't change anything.