Use a toothpick to press the hidden button inside the "AV" or "Update" port while connecting the box to your PC via a USB-A to USB-A cable.
Finding the correct firmware is frustrating because manufacturers often don't provide official downloads. Users usually rely on community forums like XDA Developers and Clube do Hardware where people share their own backups and working files. Even with shared files, users have found that not every "R29" firmware works on every "R29" board, making it a trial-and-error process. r29 5g lp3 v12 firmware