orbi cable router or plain cable router to add to orbi system?

You can't hook up an orbi router as a satellite. You could configure it in AP mode but it would need to be hardwired in. See this thread:

https://community.netgear.com/t5/Orbi/Will-The-AC3000-Router-Work-As-A-Satellite/m-p/1411118#M15511

Your best option would be to by a stand alone modem, NG CM1000 or 1100, or a Arris 6183 or 8200 or a Motorola MB7420 or 8600. Use a stand alone router with your current Orbi 50 series system. 

The CBK system is a stand alone gateway modem that only works in either gateway router mode or modem only mode. I think it would be a waste of money to but this system and use it in modem mode since you already have a good Orbi system. The wifi on the 50 series is faster than the 40 series as well. 

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quagly wrote:

I will be purchasing a cable router.  I have an Orbi RBR50 router and RBS 50 Satellite

 

I also plan to extend my network with an additional Satellite.

 

Can I using the CBR40 as a new base station and my existing RBR50 as a Satellite?

 

Or would it be better to get a cable modem with wi-fi turned off and extend my existing wi-fi with an additional RBS 50?

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Just something to consider...

It's simpler to use a standalone modem, but one important drawback is that when Orbi fails for any reason, you will lose network access until you can fix or replace the Orbi HW.  With the cable/router combo, you can quickly restore your network access and give you time to recover from Orbi failure.  It's better to have a backup that you don't always need than to not have one when you need it most.