What (Windows/Mac) are you using to prepare the USB drive? Share public link
In more serious cases, the issue isn't just the firmware but the physical memory chip (eMMC) where the firmware is stored. Firmware TCL L43S6500
(if using DVB-T2/C/S).
Keep holding the button until the screen displays "Upgrading..." or "Software Installing...". Release the button. What (Windows/Mac) are you using to prepare the USB drive
The chassis (main board) number is TPD.RT2841.PB772 . This is useful for finding highly specific service information or compatible spare parts. Firmware TCL L43S6500