Hi,
Try using the phone’s WiFi as close as possible to the router to see if it works better.
If so then there is an antenna problem. The WiFi and the BT use the same antenna as they both work on the 2.4GHz band.
The phone will have to be opened and checked that the antenna has been reconnected properly.
Here’s an image taken from a teardown video of the phone that shows the antenna cable and connection in the phone.
[image|2148764]
(click on image to enlarge for better viewing)