Phone Bluetooth extender
Bluetooth is not like WiFi where you can extend the range by adding an access point. Bluetooth is not designed for that.

You can also forget about "soldering a 2.4 GHz antenna to your phone",  2.4 GHz is RF, you can easily screw it up (and with your lack of knowledge you will) and make things worse.

The antenna is also not the limiting factor. Bluetooth is for short range. If you need long range, use something else.