Hi @nasruldeen26030
Try using the WiFi in the phone as close as possible to the home network router i.e. <10cm and check if the WiFi connection remains stable and that you get a good signal etc.
If so it may be an antenna problem e.g. loose antenna connection at either end of the antenna cable.
The same antenna is used for both WiFi and BT as they both work on the 2.4GHz frequency band
Here's the ifixit [guide|131394] guide that gives a good view of the white antenna coax cable as it goes from the top main board down past the battery (on the right hand side in the images) and connects onto the lower board.
maybe worth checking out if the WiFi works OK as suggested above