Problem with the WiFi and Bluetooth signal

Hello,

Recently I bought a second-hand iPhone 6 to restore, but I am having big problems with the signals, to connect to 2.4Ghz networks the cell phone has to be literally attached to the router, however with 5Ghz networks there is no problem. The same goes for Bluetooth to pair and use my sound headphones must also be attached to the phone. I recently ordered the F-type Wi-Fi antenna. I don't know if this will solve the problems. The board has not had water damage or dents chassis. Could it be a motherboard problem?

Thank you.