howto restore ros to rb750 from openwrt without backup flash

I installed openwrt “attitude adjustment” to rb750 without backup flash content.

Now i want to restore routeros with netinstall 6.2.

I started netintall6.2 the rb750 act led blink as usual , but rb750 doesn’t appear from netinstall window.

Netinstall can find a rb450g under my configuration.

I use tftp with rb750, it can net boot from a ram version openwrt now.

How to restore routeros without backup.

I have backuped license and serial number.