unable to netinstall in RB750

i have a RB750 (mipsbe) in 6.49.2 and i would like to use netinstall. for some strange reason the routerboard does not go into ether-boot mode and i am unable to use the netinstall tool.
i have tried to type the

/system routerboard settings set boot-device=try-ethernet-once-then-nand

without success. Any ideas, please?

The problem with quoted setting is that the lone try is performed right after interface becomes on-line. However, PCs tend to lag behind after link loss and it’s highly likely the only try will be lost. The problem is usually overcome if there’s a very basic switch between router and PC so that PC’s link doesn’t flap.