[Help] returning routerboot from ethernet (only)

Please help, how can i returning default routerboot to nand, from ethernet(only), because flashing with netinstall could not working.
it’s always coming back into netboot mode.
i’ve made script for changin routerboot mode

/system routerboard settings set boot-device=nand-if-fail-then-ethernet

but still return into etherboot.

i’ve only have utp cable only, don’t have any console cable. and i can’t open the case because it’s labelled with warranty void.

any help would be appreciate.