Asus RT-AX1800U doesn't boot from USB

I want to boot from rootfs USB, I recompiled & flashed kernel to add support for ext4, usb3, usb storage and so on.
I tried
setenv bootargs=root/dev/sda1 console=ttyS0,115200
in u-boot but this do not work, what I'm doing wrong?
When kernel boots, there is my usb hdd sda1 recognized correctly, but still old
Kernel command line: console=ttyS0,115200
Currently the bootcmd=mtkautoboot, I think that I need to set this to nand read & bootm, but do not know correct adresses.
What I'm doing wrong?

root=/dev/sda1 would be my guess.

In most cases, the bootloader provided cmdline gets ignored by OpenWrt in favour of hardcoding it in the DTS.

Ok, so where are the DTS to modify root parameter?