IT'S WORKING !!!!!!!!!!!!!! ==> GRACIAS !!hi there, i had the same problem and i came up with the solution. The real problem is the app, to update te firmware you need to install the tello app v1.1.1 (you can find it online) and a charge battery.
Without the battery press and hold the power buttom and conect the charger, keep holdin the power buttom and insert the battery, when the green led turn off release the power buttom. It will turn a pulsing red light, connect the tello to your device and procede to install the firmware from the app.
this work for me and i hope works for you.
sorry about my inglish i´m from argentina
I have been able to recover my 2 defective devices!
What an amazing solution provided by my new best friend from Argentina
If you need the Android version you can download the apk here==>: APK Link on dropbox
Working fine with my Samsung S20Ultra 5G, small error message when running but after, removed and upgraded to the latest one.