As mentioned above, there is a little Sticker on the F-Cable for the BT/GPS/Wifi Antenna. This Sticker is NOT an isolator but a connector to the frame. After experiments with aluminum foil under and/or over the the cable, to ensure connection by pressure when the phone is put back together, I found the best solution is to scatch the sticker with a sharp tool … also scratch a bit on the opposite site under the cable and on the counter side on the displays back. It is connected on both sides. Maybe use contact-fluid with a q-tip as well. It might be corroded or was touched by your hand when you replaced battery or so . Then there is no more connection possible due to oxidation or grease from your fingers. Here is a picture of the sticker: https://ibb.co/sQ4CgZP For me it works now. I am back with Bluetooth, WLAN and GPS full signal again. After half a year my BT audio works again. You need to open the phone but do not need to remove any cables. It is done in less then five Minutes. Good luck.