Why is my phone's WIFI signal so weak?

[AC Question]

1) Phone WIFI signal is weak despite changing phones (LG G2, G3, Samsung S5)
2) Phone is near router and WIFI signal is strong for my family member's phone (we switch our phones around to test it out).
3) 4G signal is strong in all instances.

Problem : WIFI signal is persistently weak regardless of phone model/location, occurring only for 1 sim card number.

Any idea why?

 

[Almeuit]

That definitely is odd. The SIM shouldn't have any affect on it. I would try other wifis to see if they're weak for the phone as well -- this way you can narrow it down to the phone or to the router. Also if you're comparing to another family members phone ensure it is the same phone -- If not please remember certain phones calculate signal differently.