Wow...I just received a phone call from a DJI rep (caller ID showed "Shenzhen Guangdong, China") who very kindly explained the 4G LTE Dongle is not being sold anywhere at present and there is no decision on when it will be for sale (anywhere in the world)
In response to some of the questions in this thread, Parrot has a 4G drone in the works, which is the only other one I've seen:
Here’s what ANAFI Ai’s 4G LTE connectivity feature will cost you
I'm not suggesting I'll go that route (still waiting for my
Mavic 3 which DJI emailed as "Shipped" 3 days ago but FedEx has only been sent the label information), but for someone who has constant signal problems due to tall oak trees, the 4G technology would be great to have.