I understand long range is illegal yes.
The arduino boards seems to be a hacky solution i would not dare to fly with it. the uav project you linked to is expensive. It seems DJI is putting there FPV system end of life its no longer sold? Which is very unfortunate.
I own the DJI FPV drone but havent tested max range im to scared haha.
The documentation lists 4km reach. I would like to have a little more reach but it has to be a solid solution.
From what I understand it uses 4G and mavlink to send the control data from groundstation to the autopilot. Then using a mavlink to dji goggles conversion and you can view the videostream on the groundstation and on the goggles.
What do all the uav projects out there use then? is it all custom build video transmission links.
Also I wonder how much bandwith 4g has to transfer lots of data.