I have 3 RB951UI with the same problem and can't find the solution on the forum.
All routers were working and suddenly start rebooting, beeping once and blanking led 4 and 3 and than reboot again every 9 sec.
I tried to use Netinstall but even using the procedure correctly it's impossible to see them into netinstall sw because continue to reboot.
Any idea on how to solve the problem?
Thanks in advance