Posting this in case it can help someone else with a similar situation.
Background: 8 Sonos components in the house. A mix of legacy Connect, Connect:AMP, a newer model Connect:AMP, several Sonos Play:1 and Sonos One’s. All on Sonos S1, all on WM:1 (House Wifi) network.
Netgear Orbi RBR50 router and two RBS50 satellites 2.4GHz Channel 1.
Everything was working GREAT for YEARS.
Then, I added a 9th speaker, a Sonos Move. This is where it starts getting weird.
Speakers start dropping off the iPhone app. My son’s iPhone app… we are looking at them side-by-side… some are on my phone, some are on his phone, some are on both, some are on neither. Sometimes they would pop back on mine and then pop off again later.
I figured it was just a software update issue. 2 weeks went by and it didn't magically fix itself.
Did adding the Sonos Move cause this? I still can’t way for sure if it was coincidence or not. Anyhow… the really weird part… ALL of the speakers are still showing on my wired Windows 10 PC Sonos app.
Sonos support had me shut off both the RBS50 satellites and… all the speakers magically show up on the iOS app.
I don’t want my satellites turned off… so, I decided to switch my Sonos network to SonosNet (channel 11) instead of Wifi.
The idea being if Sonos uses its own mesh network, the RBR/RBS won’t matter since they aren’t in the mix anymore.
So, I hardwired one of the speakers, they all switch to SonosNet (except for the Sonos Move) and the problem persisted.
I noticed the Sonos network still knew about the Wifi. Of course... the Sonos Move is still on Wifi. Maybe they are all still communicating with the RBR and RBS, even though they are supposed to be on SonosNet?
By powering down the Sonos Move, I gained the ability to remove the SSID from the Sonos configuration. I thought maybe this would work, with everything on SonosNet and Sonos having no knowledge of my Wifi network.
Everything appeared to be working fine, albeit without the Sonos Move.
(I did not test this configuration for any length of time without the Sonos Move. My suspicion is it would have worked forever, but I wanted the Sonos Move to work also.)
I factory reset the Sonos Move and tried to add it back to the network without Wifi. It requires Wifi. No SonosNet for Sonos Move... Bummer.
So I started looking for other solutions and came across the Netgear Orbi RBR50/RBS50 firmware update list. I was on an old firmware version. Release notes state that version 2.5.1.8 "Fixes the compatibility issue with Sonos devices." SWEET! The latest was 2.5.1.16, so upgraded to that version.
Fingers crossed... everything seems to be working at the moment...