ok so to start
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open the case and record all chip models
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Identify the UART pins (you did already)
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study a similar commit to make a similar DTS and profile
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build an
initramfs-kernel.bin
and load that over TFTP to boot (NOT flash)
here is a commit for a mostly similar board for example
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=dbaf0d20bb6afec4a592456af6b1238b46fa1acb
generic building guide
to build initramfs-kernel.bin
in ramips
in make menuconfig
after selecting Target Profile
select Target Images
enable ramdisk
edit: before going through the build process see if there is a command or key for loading TFTP in the bootloader
you can paste your serial logs here