Why is my Wi-Fi so slow when I'm within range?

I recently got Xfinity, and for the first month everything was fine. But now, I can't load anything on my devices unless I'm within 15 feet of the router. I can be further away, and get 2 or even 3 bars on my iPhone, but no pages will load. I'm well within range, but something's wrong.

It's an expensive Netgear router, and I have high speed internet, so I know nothing's "wrong" with the router. I've tried to call Comcast but all they tell me to do is reset it. I've reset the router in as many different ways as I possibly can. Also, sometimes it will work perfectly for a day or two at random, and then go back to being terrible. I find it obnoxious that I'm paying for high speed internet, and yet can't even use the Wi-Fi in half of my house.

Could I be doing something wrong, or maybe need to change a setting? Ethernet and wired connections work perfectly, it's just Wi-Fi. Maybe it's just something on Comcast's end that they've screwed up but they're no help.

If you have many Wi-Fi networks around you it might be that your Wi-Fi channel is overlapping. I would change channels in the defualt gateway, 192.168.x.x.x look up your defualt gateway