Is there any system that works for at least 10-20km without line of sight, supporting a video feed usable for guidance, without cell phone network coverage?
Maybe something where there are two drones, and one flies high with line of sight to both the user and the low-flying drone so that it can act as a relay like a satellite would?
Or something that uses radio frequencies that bounce over the atmosphere, and if bandwidth is not enough uses an automated pilot plus the ability to send photos every few seconds so the user can tell it in which general direction it should fly?
Or something that uses Starlink/Iridium/etc. to relay data?
Those things are all possible, but you’re not going to find it in a COTS product.
The key terms in the link budget is directionality. If you use a phased array (like Starlink) or a parabolic dish, you can boost your range tremendously. This is how 4x4 MIMO WiFi can achieve gigabit speeds with the same power limit.
I think it would be very useful for scouting in woodland/backcountry use including search&rescue.
At 50 km/h, it moves 10-20 times faster than a hiker, and at <1000$ it's more than 100x cheaper than a plane or helicopter.
But requiring line of sight is a strong limitation because it means that you need to fly over the tree canopy and might not see under it, and you need to fly really high if target is on the other side of a mountain or hill and again you might not see well.
And of course cell phone coverage is not guaranteed so you can't use the easiest solution of putting a cell modem on it.
In reality you will see about a 2km max in relatively clear conditions, and as few as half a km in even normal suburban/urban conditions.