After having struggled for weeks with my HK devices and (new) Orbi router, I posted the following on Netgear's community yesterday:

I'm having some issues with some devices staying connected to Homekit, including Hunter Fan, Nanoleaf & Meross bulbs.  Still other devices have had no issues at all, including Circle View doorbell, Kasa plugs and VOCOlink light strip
Per recommendation, I disabled 5Ghz and was able to add the Hunter fan to the app and HK, but days later I noticed streaming services being pixelated so I re-enabled 5Ghz for streaming Apple TV.  Two days later - probably when the DHCP Lease renewed - the fan became unresponsive, as did the Nanoleaf bulbs.  The Meross bulbs worked fine on the Spectrum router, but drop minutes after being added now on the Orbi router.
Can I do anything to allow both bands to be up, but assigning specific devices to 2.4Ghz only?

I was advised that there was a firmware available (which did not automatically download/update) for my router which adds IoT network functionality. I'm not quite sure how to best utilize the functionality though, given my understanding of HK (#1) and my performance needs (#2).

So based on my understanding of HK:
a) many HK devices specify they only work on 2.4Ghz only
b) my iphone must be on the same SSID as the HK devices
c) my HK hub (AppleTV or HomePod (I have one of each on opposite ends of the house)

The AppleTV took a performance hit (pixelated streaming of content) when I had 5Ghz disabled, and re-enabling it knocked at least three devices to be unresponsive in HK (and no longer present listed as connected devices to the router).

How does a separate SSID for IoT Network help here? Is there a best practice you all have found?