RBR750 + RBS350 connection problem after FW upgrade

Try this to see if this works:
What you need to do is factory reset the RBS. Wire connect a PC to the 1st satellite📡. Already have the FW version files downloaded on to this PC. Use MS Edge or Firefox for this. 

You'll need to turn OFF the RBR to ensure the RBS don't connect to the RBR. Wire connect 1 PC to the back of the RBS. Factory reset the RBS, wait about 5 minutes. The PC will get a IP address from the satellite📡or try setting a static IP on the PC for 192.168.1.49. The RBS will be 192.168.1.250. Use this IP address in a web browser to log in to the satellites web page. Log in as admin and password. Use the satellites 📡 web page FW update feature to select the FW update file that was unpacked from the .zip file. Then factory reset and then re-sync the satellites 📡 to the RBR. 

Or try TFTP to get the RBS back to the lower FW version:

https://kb.netgear.com/000059634/How-to-upload-firmware-to-a-NETGEAR-router-using-Windows-TFTP
Need to press the reset button for 30 seconds then release, then it send to PUT the command and upload the FW. 
Make sure you enable tftp: https://www.thewindowsclub.com/enable-tftp-windows-10
Video walk through using a tftp client. Use the built in one for windows 10 though. the video is more of an example of how its done:
https://www.youtube.com/watch?v=hXhMgQS5k1o
https://www.youtube.com/watch?v=R9DLrLQrpaw
Mac TFTP: https://www.petenetlive.com/KB/Article/0001247

Try nmrpflash:
https://github.com/jclehner/nmrpflash