iMac won't join network

What FW is currently loaded?
What is the Mfr and model# of the ISP modem the NG router is connected too?

What channels are you using? Auto? Try setting manual channel 1, 6 or 11 on 2.4Ghz and any unused channel on 5Ghz.

Try disabling the following and see:
MIMO, Daisy Chain, Fast Roaming, IPv6 and Set 20/40Mhz Coexistence to 40Mhz only. Save settings and reboot the router and satellite(s).
If you updated to recent FW v.210, try enabling Daisy Chain. Some have mentioned that this seems to be working in reverse order, enabling means disabled actually. Hoping NG support can have a look at this. So try this out as well.
https://community.netgear.com/t5/Orbi/Firmware-2-2-1-210-and-connection-problems-Skybell/m-p/1649275/highlight/true#M43001

Try this as well:

Advanced Wireless Settings both 2.4GHz and 5GHz changed CTS/RTS Threshold to 2307.
https://community.netgear.com/t5/Orbi/Orbi-RBR40-RBS40-V2-1-4-16-mesh-network-drops-connection-to/m-p/1601598/highlight/true#M35230
Change the CTR/RTS threshold values for both 2.4 GHz and 5 GHz (2341 and 2345 respectively), changed preamble modes to long preamble, and Set 20/40Mhz Coexistence to 40Mhz only.

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jmielke wrote:

imac running mojave immediately says it can't join the network when the SSID is selected. Sometimes it asks for the password and then says it can't join the network. The machine can join other networks; just not the Orbi.

 

 

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In my case such "starnge" issues -I noticed- were not directly related to the device or Orbi but were caused by "loops" in my network!

Please disconnect all wired devices, restart Orbi and see if the issue will be gone. If yes, then start connecting the devices one by one to locate which device is causing the issue. If you have ethernet mesh speaker (Sonos for example) then most probably it's them causing all your troubles.