Try WinBox with the "Neighbors" tab (besides "Managed"), when connected to the same network as the RouterBoard.
If the Routerboard shows up, connect via the MAC address. (Click on MAC address it will be copied to the "Connect To" field.
When logged in with the same user account , try the top-left 'undo arrow' , it might just restore the default gateway. (but you rebooted already
, that redo/undo stack is probably gone)
"Safe Mode" is there to help you with configurations that cause connection loss, by rolling back if connection is lost.