Set up reserved IP addresses? 

If three devices are connected via Ethernet, most of your system isn’t connected to WiFi except for the Moves.. The older speakers will be connected to SonosNet, which gets generated when you wire one or more Sonos devices. 

Since you’re running Unifi, this may be of assistance: https://github.com/IngmarStein/unifi-sonos-doc

Sonos released a faulty firmware in June 2023 that causes disconnects and drop outs in some units. You should contact support to see if you have players affected. 
 

edit regarding UniFi, if you have UniFi switches and router you should check out  Bruce’s link.  But if you have UniFi APs and unmanaged switches, Sonos’ use of stp shouldn’t conflict with your aps (ime, also own 2 UniFi APs that work flawlessly)

External factors can also come into play. WiFi networks are more prevalent; a neighbour may have set up their network on the same channel as yours; other electronic devices may be causing interference… Reserved ip addresses and ensuring my wifi was on a fixed channel solved my issues, except when a neighbour chose the same channel as me for his wifi - but that was easily fixed!

The external factors can be a pain, we just got Quantum Fiber in the neighborhood and I’m seeing most of the Cox Cable WiFi SSIDs disappear replaced by Quantum ones. Worse the Quantum folks are handing out three node mesh systems for free with every install.

Eight neighbors, six have moved and I’m on the list for tomorrow. Then I’ll try to sort out both the 2.4 and 5 GHz bands and see if I can keep my Sonos (on SonosNet) happy. Worst case I’ll dig out my Boost and wire a couple more to Ethernet.

Moved over to Quantum Fiber several months ago and just this week worked to re-setup my Sonos system.  My Sonos system up and running with some caveats. My Sonos system is small and old, but is now operational on Quantum Fiber hardware.  Below is the hardware and a short list of what worked and what did not work.

1. Quantum Fiber: Quantum provided a C6500Xk Fiber Modem & Router and three Q9500XK TRI-BAND WiFi Pods to provide their 360 WiFi (these PODs are Tri-Band).
2. My Sonos: 1 Play:3 speaker and one Play:1 speaker. Note: both these speakers are Sonos “S2 App” compatible hardware. 
3. My home network uses both hardwired components and WiFi hardware - all on the same  192.168.0.xx network.  NOTE: two different hardwired PCs could not EVER find an existing  Sonos system to join as a controller even though I could successfully ping the speakers from the PCs.
4. Experienced many failed attempts trying to use the PC to setup the speakers as a Sonos system when the speakers were on the WiFi network and the PC connected via cable.  The PC could see ping the speakers and the C6500xk device list showed the speakers and the PC in the active device list on the router - all with 192.168.0.xx IP addresses.
5.  In a test with the speakers hardwired, I could setup a Sonos system with my PC as the controller.  But in this configuration could never get a Wireless device to join as second controller nor could i use the Sonos `hardwire to WiFi conversion process’ to add a mobile controller to the system.  I need wireless for my home application so this test was torn down. There is a clue here as to the failure of hardwire vs WiFi devices to see each other - but someone with more network knowledge will need to tackle that issue.
6. WiFi Setup that worked:
   1. Did the factory reset to default values on both Sonos speakers
   2. Used the Sonos S2 app installed on a Samsung S22Ultra.  Powered only the Play3 speaker.  The The S2 app took a long time to detection the speaker - the app stated nothing was found and then 20-30 seconds later, the speaker showed up and successfully was add to the Sonos system.
   3. Powered up the Play1, the S2 App applied a an upgrade (firmware or Software??) which was successful.  Using the S2 app to add the Play1 speaker, experienced the same detection delay.  The S2 App stated that the new device was not found and 20-30 seconds later it was recognized by the app and was successfully added to the Sonos system,
   4. Using the S2 app on a Samsung WiFi Tablet, successfully add the tablet as a second controller.
   5. Still an issue: Neither of my hardwired PCs can find this successfully working Sonos System to be added as another controller.

@Nail59A,
Note the PC App has been in ‘maintenance mode’ for quite some time and no longer has administrative features to setup products. As you’ve discovered, you need to use either the Android or iOS mobile controller App to do that. 

Your PC Sonos Apps though should work for playback/control.. I would perhaps uninstall/reinstall those and ensure the firewall permissions are correctly put in place, when prompted - and certainly ensure the network connection for the PC is also set to ‘Private Network’ rather than the default ‘Public Network’ and see if the ‘existing’ Sonos system is then discovered on the same LAN subnet.

The internal router in the Quantum NID (5500 or 6500) is an adequate but not outstanding device, the 360 Pods are reported as working well by several neighbors, but none have Sonos.

I’d keep an eye out here and in other Sonos / Quantum related forums to see if someone comes up with a set of optimal settings for the combination.

I’m using the 5500 NID in transparent mode along with my own router and WiFi and so far have seen no issues.