Hi @leimalimban
Try using the phone's WiFi as close as possible to your home network router i.e. <15cm and check if the WiFi detects, connects and maintains the WiFi connection OK.
If it works OK, then it may be a faulty antenna/antenna connection that is the cause of the problem.
Here's an image taken from this teardown [https://www.youtube.com/watch?v=dMDn5ZJK_iQ|video] of the phone, showing where the antenna is connected.
[image|3662690]
(click on image)