Sounds like an SSDP Multicast discovery issue where your controller device loses sight of your Speaker(s). It’s difficult to know precisely what the cause might be, But…

I’ve seen this happen with some network setups that have multiple WiFi access points/hubs operating on different channels and/or SSID’s and the Sonos controller connecting to one of those AP’s cannot discover the devices, even when they’re wired, as the multicast packets cannot get through .. It may also happen where Speakers are wired to powerline adapters, or there is a lot of interference surrounding the network.

You might need to describe your Network setup in more detail for others in the community here to suggest some things to try. Maybe include things like channels in use (both SonosNet/WiFi) and channel width, details about modem/router WiFi Hubs (if using a mesh setup) other access points, switches etc.

Seems odd. You’ve done many of the normal things I’d recommend, minus one. I’m wondering if the Arc itself is in a “weird” state and needs to be powered down for a few minutes, then plugged back in. 

If I were to guess on a normal setup, I’d say your Arc has a duplicate IP address issue, at least on the face of it, but you state that everything already has assigned IP addresses. So I’m wondering if the firmware on the Arc itself needs to be restarted from “fresh”. It working in some ways, as it’s passing the fact that your Sub and surround speakers are connecting to it (I’m assuming they’re not wired) and the router is showing their IP addresses. 

However, this is definitely an odd state. If a power cycle doesn’t resolve the issue, I’d be very tempted to submit a diagnostic when it is showing up in the Sonos controller, and contacting Sonos directly. There might be information in that diagnostic that points to why this oddity is occurring. 

Edit: I see some good info from Ken that’s worth investigating, for sure. 

Sure,  thanks for the suggestion.

I use a Netgear Orbi tri-band router that connects to a pair of satellites.  I have had this setup for a long time with minimal Sonos issues.  The Arc is the S2 root zone and is directly wired by ethernet to the main Orbi router which is about 5 feet away. 

The S1 system (which works perfectly) is connected by ethernet to one of the satellites through my Gen 1 Play 5. 

S2 is on Ch 1, S1 is on Ch 11, Router is on Ch 6. I do not have multiple SSIDs. I do not use powerline adapters or extenders. 
 

I fear I am going to have to do a factory reset which means I have to set up all my services,  Truplay, etc.

Are you able to wire an S1 stand-alone speaker direct to your main Orbi Hub - rather than having the Play:5 (gen1) wired to a satellite Hub.. it’s best if both S1/S2 systems are wired to the main router/Hub. You just need the one device only wired for each setup (S1 & S2)… I’m assuming the Orbi Hub is set as a router and not ‘bridged’ off some other router, in which case, the setup would need to be altered.

I can’t see much wrong with the overall, setup other than what I’ve briefly covered here - just ensure the wired Sonos Products are 1 meter away from the main Orbi Hub and from each other.

You may want to help things a little too, by reserving your Sonos IP addresses in the routers DHCP reservation table and if you do not have any portable products (Roam/Move) then also remove your routers WiFi credentials IF they are stored in your Sonos App Network settings - they are not required when running all on SonosNet.

Those would be my suggestions for you to perhaps consider in order to try to resolve the problems you’re seeing.

I did notice that my Wi-Fi creds were showing in the S2 app, probably from the original setup. I removed those and rebooted the Arc. It grabbed it’s reserved IP and so far seems solid.  I’ll let y’all know. 

Still having some issues. Some new observations.  When the Arc is recognized in the app, it is given the IP 192.168.1.38 which is the correct reservation for it.  When I look in my router at the attached devices, the Arc is not listed,  all the other Sonos devices are,  both on S1 and S2.

I can ping it from a laptop, and it shows n the network matrix.  

Being that it’s Thanksgiving,  I’ll muddle through today and call Sonos tomorrow and send a diagnostic.