iPhone 6 - bad wifi connection

hey, I have an iPhone 6 which has been working great up until now.

when I connect to wifi, it shows that I am connected but whenever I try to surf the Internet or use anything like an app for instance that works with Internet connectivity, it doesn't seem to work at most times. Rarley does it ever work perfect like it used to.

Ive contacted my service provider and they've told me my connection is fine which it is because on all my other devices, the Internet does work.

what options do I have to troubleshoot this problem aside for resetting my network settings which I've done and didn't make a difference.

ive turned my Bluetooth off as well and still nothing! Starting to get extremely frustrated.