WiFi connection is different on certain devices.

Recently my iphones (iPhone 6) wifi connection has been really weak. Barely hovering over two bars no matter how close I get to the router. Sometimes it disappears entirely if I walk into a certain room. This doesn’t seen to be a problem with my other devices like my laptop, iPad, iPhone 5, etc. Other times when I visit friends, it won’t let me connect to their WiFi either, the message “unable to join” will always appear after trying. I’ve tried almost everything, resetting, manually setting DNS, restarting router.