bought new rbs750 cant sync

You may be in a "Catch 22" situation. Netgear recommends that satellite firmware should be updated before updating the router firmware.  (My sense is that if the router is updated first and there is some incompatibility between router and satellite firmware, then it will be impossible to update the satellites after the router reboots to the new firmware.  If the satellites are done first, they may simply wait for the router to reappear with firmware that they recognize.)

If a 'wired' connection allows you to learn the IP address of the new satellite, and you can find a way to get access to the satellite web interface, then the new firmware could be manually installed on the new satellite.

If there is simply no way to get into the satellite, then there appear to be two ways forward:

• Downgrade the router firmware to match this new satellite. Update the satellite. Then update the router again.
  (What I cannot figure out is how to know exactly which firmware version is on the satellite.)
• Use TFTP or nmrpflash (https://github.com/jclehner/nmrpflash ) to load the newest firmware directly onto the RBS750.  For instructions see:
  https://kb.netgear.com/000059633/How-do-I-upload-firmware-to-my-NETGEAR-router-using-a-TFTP-client-on-Microsoft-Windows 
  I have done this, and it is not for the "faint of heart".  Having failed five times in a row, I was ready to take a hammer to the box and thought, "what the hell. what else do I have to do right now?" and the sixth time.... it took!
  Apparently there is a tiny window of time when the device looks for a TFTP connection. A second too soon, and "no go". A second too late, also "no go".