Default HTC One Google Play not connecting to mobile network

Hi,
All of a sudden one day my phone stopped connecting to any mobile network. The signal strength is stuck on 0 dBm 99 asu no matter what I do, and factory resetting it doesn't help either. It also takes now a long time to boot (and the opening animation seems to stuck for a long time), but I don't know if it is connected.
Any help?

Thanks.