WiFi, visible light, and X-rays are all just electromagnetic radiation. The higher the frequency (or shorter the wavelength) the higher the energy per packet. In order for EMR to give you cancer, according to theory at least, the energy per packet has to be high enough to ionize atoms in your body. Specifically, in your DNA. This causes an error that, most of the time, does little or nothing. If you're very unlucky, the error will happen in a particularly bad spot and cancer can result.

It's important to note that the energy per packet rather than the number of packets that matters. It's similar to how I could fire a thousand ping-pong balls at you and you'd be fine, but one bullet would be a completely different story.

X-Rays are very short wavelength and have a relatively high probability of ionizing atoms in your body. UV light is longer in wavelength, but is still just barely ionizing. The lights in your home are probably pretty safe. WiFi is substantially longer wavelength (and lower energy per packet) than visible light, so it's even less likely to give you cancer.

Put simply, if you're worried about getting cancer from ionizing radiation you should go live in the dark in a cave as far underground as you can get before worrying about what's coming out of your cell-phone or router.