1. Go to Call menu, dial *#*#4636#*#*, pick your Simcard (1 or 2) and set the Preferred Network Type to TD-SCDMA, GSM/WCDMA and LTE
2. In our forum, a user said he tried to use the phone without simcard, use 1 slot, use 2 slots, and combination amongst it and suddenly it worked. Maybe you can try it.