My phone can't connect to the Wi-Fi?

Everyone else in my family has Wi-Fi on their devices, but somehow, only mine is unable to connect. I've tried everything from hard reset, disabling and enabling SSID, turning the router and device on and off and I even tried unplugging it from the plug. Does anyone know how to fix these? Could it just be because my devices are old? I own an iphone 4s and a 7" kindlefire HD tablet.

Could be a bunch of reasons. Someone blocked you from using the router, it's setup as a 5Ghz only router and your phone doesn't support it. Problems with the area of the house you use the devices. And on and on.