What version? Sounds like your mobile data connection when you switch from WiFi to mobile. There was some issues early with lollipop...Nexus 4. Wondering why I have to shut down phone every time I go out. The triangle always shows an "!" and get a message it's offside. Powering down all the time is a pain.
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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