Hi,
The Bootloader code is integrated into the ROM of LPC5504, I think you can use use bootloader to program the LPC5504 on-chip flash.
The PIO0_29 and PIO0_30 are Uart pins, you can connect the two pins to a USB-UART board from third party to do the work.
On the PC, you can use blhost command.
This is blhost command instruction UG.
https://www.nxp.com.cn/docs/en/user-guide/MCUBLHOSTUG.pdf
You can download the SDK from the link:
https://www.nxp.com/design/software/development-software/mcuxpresso-software-and-tools-/mcuxpresso-s...
After you unzip the package, pls refer to the directory for the blhost.exe
....\SDK_2_10_1_LPCXpresso55S06\middleware\tfm\tf-m\platform\ext\target\nxp\common\secure_boot\tools
Another tools is Flashmagic, you can download application code via both SWD and in ISP mode.
https://www.flashmagictool.com/
Hope it can help you
BR
XiangJun Rong