K790a Coverage Problems

Hi i'm new to this site and i'm looking for help. I have K790a unlocked and i believe debranded, the phone was originally locked to a service provider i got it unlocked but when i use the sim card of my new service provider it tends to loose coverage and when i search for a provider i get the option of the old provider i'm not sure what to do some one any one has any ideas

If your phone is debranded you haven't any icon from old operator,but if your phone is just unlocked - you have (maybe that is problem),so if you have that icons you need to debrand phone.Use this tutorial XS++ 3.1 Guide-Flashing, Customizing, Recovering - Top Sony Forums and when you connect your phone to XS++ post log here without IMEI number