why does my signal go in and out?

[AC Question]

I've a lg g4 815 model. Initially when I bought my phone it didn't have this issue but 4-5 days after upgrading to marshmallow while I was driving to my office the signal was lost while still talking on phone. In the city near my home I get signal but not full (whereas on the other phone with the same SIM I get full signal) and at my office I don't see any signal. I changed the SIM even and still I have the same issue. I even did the factory reset and installed lollipop back and it still didn't resolve the issue. The*#*#4636#*#* I did but when I click phone information it says the app doesnt work on this device. Plz help me into this guys. Really struggling with my phone.