Unable to add two additional satellites (RBS750) to existing RBR750 setup

Best way to hopefully get the incoming RBS to connect to existing system, would be to setup the 2nd RBR and RBS, off line from the current system and update the RBR and RBS so that they are on same level of FW as the existing system. No need to connect the RBR to the ISP modem. Leave the current system running. I'd turn OFF the existing RBS to better help get incoming RBS connected at a later point. 

Have the RBR and RBS v.31 FW files already downloaded to a wired PC. 

With the 2nd system, setup the system using a wired PC and web browser:

https://kb.netgear.com/22697/How-do-I-install-my-NETGEAR-router-using-the-router-web-interface

Bring the 2nd set of RBS to same room or near the 2nd RBR. 

Allow the setup wizard to manually setup the system. Can follow this PDF process:

https://community.netgear.com/t5/Orbi-WIFI-6-AX-AND-Wi-Fi-6E-AXE/Procedure-for-Factory-reset-of-RBS850/m-p/2356834/highlight/true#M52575

Skip pass the factory reset point the the PDF and where your waiting for the setup wizard web page to appear in the web browser. Then continue the process from there. 

After the 2nd RBR and RBSs are setup and connected to each other and your at the home page for the RBR, manually update the RBS to v.31 FW, then update the 2nd RBR lastly to v.31. This PDF process gives details on how to manually update an Orbi system:

https://community.netgear.com/t5/Orbi-WIFI-6-AX-AND-Wi-Fi-6E-AXE/Procedure-for-Factory-reset-of-RBS850/m-p/2359585/highlight/true#M53030

Once the 2nd Orbi set is fully updated. I believe you should be able to power OFF the 2nd RBR. I'd power off one of the 2nd set RBS leaving one ON. While the one is ON, the RBS will begin to blink or turn PINK in front. Log into the 1st RBRs web page with a web browser and use the 1st RBR Add Satellite feature to add the 1st coming RBS to the existing system. Hopefully should sync and the front RBS LED should turn BLUE. After 3 minutes, turn OFF the 1st incoming RBS and turn ON the 2nd RBS. Give a few minutes then run thru the RBRs Add Satellite feature again to get the 2nd incoming RBS connected. Hopefully this connects as well. 

If both incoming RBS syncs fully to the existing RBR, then you can power them off and then go place them in there remote locations then power them all ON one at a time. 

What is the size of your home? Sq Ft?
What is the distance between the router 📡 and satellite(s)🛰️? 30 feet or more is recommended in between RBR 📡 and RBS 🛰️ to begin with depending upon building materials when wired or wirelessly connected.