This site frequently hosts AKAI and LDD board firmware. Search their database for "AKTV5537S" or the board number "LDD.M538.B138". Firmware.VIP: A common repository for 4K Smart TV firmware.
The firmware files for the (mainboard LDD.M538.B138 ) with a 3840x2160 resolution are typically hosted on specialized electronics repair and firmware archives. This site frequently hosts AKAI and LDD board firmware
Copy the extracted .bin file to the root of the USB, plug it into the TV while unplugged from power, then hold the Power button while plugging it back in to trigger the auto-update. This site frequently hosts AKAI and LDD board firmware
Ensure your TV's internal RAM/Storage matches the 5GB/4GB (or 1GB/8GB variant) specified in the file name, as flashing the wrong partition table can brick the device. This site frequently hosts AKAI and LDD board firmware