The source found!
5G coronavirus conspiracy theory: A toxic cocktail of misinformation
We found that there was a specific Twitter account, set up as @5gcoronavirus19 with 383 followers, which was spreading the conspiracy theory and had become influential in driving it forward on social media. The account was able to send out 303 tweets in seven days. We also found that President Trump was often tagged in tweets and was influential within the network without having tweeted himself. This highlights the point about support for these theories coming from the alt-right.
(Not sure if Trump has actually been taking part in spreading the conspiracy theory.)