Hi, I am A. Stevens here to help with your question! If you've already tried refreshing the software and also changing the SIM, I would have to recommend that the phone's internal antenna is problem faulty...have you had it less then a year? Google/Nexus can replace the unit for you if you let them know you've already tried everything you mentioned
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You could try replacing this antenna cable: http://www.ebay.com/itm/like/331389186871?lpid=82&chn=ps
Could also try following this video: https://www.youtube.com/watch?v=K1-uCIb-t8I
Sorry I wouldn't be able to directly fix the phone for you but all I can do is offer suggestions on how to possibly fix it...