i have the same problem but after opening the phone and change the wifi antenna only, so its definately a hardware issue. Everything else is the same so it's not the battery. i have opened/closed the phone many times so i would say its maybe connector 7, i may damaged it and thats will be my next step.
i also noticed that if i close the phone without placing the 7 screws on the motherboard (so the motherboard is not too tied to the back) i may have 1 or 2 bars. in may occasion i have the "searching" up left in the screen but i can send sms and internet browsing but no calls!
hope i helped a bit