While there are various things that can be done to improve the video quality, the bottom line is there is no way to make it reliable. This is simply the nature of the system, relying on streaming over wifi. Wifi, as a transmission media and protocol, was not designed to reliably stream from a moving target.
This is why DJI put the millions of $ into developing Occusync, used on the Mavic Pro. It achieves what you all are looking for, over miles distance.
Even with a transmission technology designed for reliable transfer of the image, there are still too many variable beyond the control of the system to ensure quality footage, so data is captured locally on and SD card.
Bottom line, the Tello
is a toy. Period. I have one, and have loads of fun harassing Duke my labrador with it. However, if I want to do anything slightly more serious with video or images, I go straight to the Mavic Air or Pro. The Tello
just isn't made for it.