Did you install the rescue system to flash?
If you didn't, the original OS is still on the flash. The first chance to uboot env, gave a back_to_stock command to change back to stock.
Now you say you have flashed the modern uboot, but be examining the original back_to_stock and you'll see how it works, and should be able to do the same on your modern uboot.
$this->bbcode_second_pass_code('', 'bootcmd_stock 'setenv bootargs $(bootargs_stock); nand read.e $(loadaddr) $(kernel_addr) 0xA00000; bootm $(loadadr)'
bootargs_stock 'console=ttyS0,115200 mtdparts=nand_mtd:0x100000(uboot),0x80000(uboot_env),0x80000(key_store),0x80000(info),0xA00000(etc),0xA00000(kern
el_1),0x2FC0000(rootfs1),0xA00000(kernel_2),0x2FC0000(rootfs2) root=/dev/nfs rw init=/init'
')
But first you need to access uboot - and as it sounds like you can't access arch, that will have to be direct across the serial interface. On the NSA325 this is fairly easy, when you open the box, there is a 3.3V UART with pin header already mounted. So with a cable like FTDI RPi you can access uboot, and try the commands.
If you did flash the rescue system, I *think* I have a copy of my original flash I can send. This said I have never tried it, only took a copy years ago, and never checked.