20 years is not the correct period.

Wi-Fi wasn't mainstream back then and kids didn't have phones in their pockets all day long.

I believe we will have more accurate data in another 30 years.

Cancer is a very broad church. There have been a lot of lifestyle changes over the last twenty years, very difficult to specify control groups ....

You could possibly use the Amish as a control group. They are probably exposed less than almost any other group to these types of radiation due to their distance in proximity. They, of course, do come into contact with it in minor ways when interacting in public, but far far less than the general population.

That's actually a bad control group; you want some group whose lifestyle, including lifestyle changes, matches pretty well the rest of society but for exposure to the kind of radiating devices at issue, to isolate changes due to the effects of the devices at issue from the effects of other lifestyle changes.

Since the Amish lifestyle and lifestyle changes over the time period of interest don't match the general public in ways other than cellphone/wifi exposure, they aren't really a good control.

The Amish live in a very different environment from the general population in all sorts of ways. So they're not a good control group, because where their health differs from the general population, you don't know if it's because of radiation exposure, diet, electric lighting, chemicals in cars, or any of a thousand other things.

The Amish are a bad control group because they have a very small gene pool among other reasons.