The thing is that Internet access is a factor that is difficult to distinguish from other potentially more significant factors such as wealth and security of a country.
To prove that the Internet is independently a determinant of JW activity you'd need to find poor/insecure countries with high Internet access and vice versa to test it out.
Pippa Norris showed that religiosity generally is higher in counties with low security and development.
https://www.amazon.co.uk/Sacred-Secular-Religion-Worldwide-Cambridge/dp/1107648378/
I suspect JWs follow the same pattern as outlined in this study, although I've never done the numbers.
In fact I think I can think of a counter example. Internet access has taken longer to grow in Germany than the UK for example, despite Germany being more prosperous. Yet JW growth has been more sluggish in Germany over the past two decades than in the UK. It's weak evidence either way, yet if the Internet significantly slowed growth we might expect the opposite pattern, ceteris paribus.