The unique form that i know for improve the video quality is increase the "Bitrate = 4mbps" before each flight. (if the tello aircraft is far, the quality will be poor and have "skipframes")
I advice you see the next video:
Yes, you need first connect the Tello aircraft with your smartphone. Then in settings (option "more") should appear that:
So, change the altitude limit. I advice you 30m like MAX vertical distance. And check always the signal icon of the telemetry.