I've found that, once you enter mobile network settings, and apn settings your mobile data connection will come back without restarting your phone. I have also same issue that start occuring after 5.1 update.
Wysłane z mojego Nexus 4
We all have got hope. That would be nice from Google to get brand new 5.0 Android on our nice Nexus 4. Anyways if Google isn't preparing Android L update for us. We will get it from hamerhead port as soon, as Google publish final version of rom.