Maybe the previous user have switched the NetBoot RouterBoot code from bootp to dhcp, like i did when trying to install OpenWRT to my RB750GL.
I figured out how to switched that settings back with some Linux utils and to get netinstall-cli to reinstall my original firmware.
But how to do it you have to have some Linux knowledge.
Here are my old thread how I did it.
And there are a new way of extracting the the device netinstall code from the netinstall64.exe see this github repo: