rb750 not detecting rbs750 satellite

Are you syncing both RBS one at a time with the RBS in same room or near the RBR? I would do this first. 
Try syncing the problem RBS with the other RBS turned OFF.

I would also generate a new backhaul code under Under Advanced Tab/Advanced Settings/Wireless Settings

What CAT# lan cable are you using between the RBR and RBS? 
Any LAN switches in between as well? If so, Brand and model please.

What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too?
Be sure your using a good quality LAN cable between the modem and router. CAT6 is recommended.

Has a factory reset and setup from scratch been performed since last FW update? A complete pull of the power adapters for a period of time after the factory reset then walk thru the setup wizard and setup from scratch with a wired PC and web browser. Recommend setting the default DHCP IP address pool range to the following after applying and a factory reset: 192.168.#.100 to 192.168.#.200.
https://kb.netgear.com/24089/How-do-I-specify-the-pool-of-IP-addresses-assigned-by-my-Nighthawk-router
I would power OFF the ISP modem for 1 minute. Factory reset the Orbi router and power it off. Power ON the ISP modem and let it sync. Then power ON the Orbi router and walk thru the setup wizard again using a wired PC and a web browser.
Press the back reset button for 15 seconds then release. 
https://kb.netgear.com/31486/How-do-I-reset-my-Orbi-system-to-factory-default-settings
https://kb.netgear.com/000062081/How-do-I-erase-the-configuration-settings-on-my-Orbi-WiFi-System
https://kb.netgear.com/9665/How-do-I-perform-a-factory-reset-on-my-NETGEAR-router