Use the Asus Recovery Utility. No disassembly or soldering necessary.
Open the utility on a Windows computer. Point to the OpenWrt sysupgrade file. Connect the unit with an ethernet cable directly to the computer. Plug the unit in the wall while holding down the reset button. Release the button when the “led bar” light up (or blink). The flashing process then starts. You can do the same to revert to the Asus firmware.
To install OpenWrt on RP-N53 you need :
Please choose the operation: 0: Load System code then write to Flash via Serial. 1: Load System code to SDRAM via TFTP. 2: Load System code then write to Flash via TFTP. 3: Boot System code via Flash (default). 4: Entr boot command line interface. 5: Load Boot Loader code to SDRAM via Serial. 7: Load Boot Loader code then write to Flash via Serial. 8: Load Boot Loader code to SDRAM via TFTP. 9: Load Boot Loader code then write to Flash via TFTP.