Hi @dinsulaeman
Try using the phone ''as close as possible'' to your wireless router i.e. <10cm and check if the WiFi detects and connects OK.
''If it does'' then there may be a problem with the [https://www.a-accessories.com/blue-wifi-antenna-coaxial-cable-rog-phone-87342-77725.htm|WiFi coax antenna] in the phone e.g. loose or faulty perhaps. (example only to show what the antenna looks like)
I can't find a teardown video for your model but here's a video for a [https://www.youtube.com/watch?v=SgBOvBIvEaE|ROG 5s Pro] that may help. The blue antenna can be seen (one end anyway) at 7:05 minutes into the video.