This is often repeated, but I wonder if it's completely true. I'd completely understand if it was harder, and more expensive, to find such a control group, but it surprises me that it would be "impossible".

I can think of a few people I know that are much less exposed to 2/3/4g radiation, by living in the forest and not having a phone on them all the time. Isn't a control group that has been exposed to significantly less radiation still useful ?

The Sentinelese, but they shoot visitors.

There are large cellular areas not exposed to significant (read: detectable) levels of radiation if you're willing to travel. Third world Countries would be a good place to start.