Is the Amp, Port, or Play:5 close enough to the router where you can wire it with an ethernet cable?

No but that's not the point. its clear the App has issues.

Thanks 

If it’s that clear, why do I not have issues on three separate iOS devices, across three different households?

You are lucky. If you Google ‘ Problems with Sonos App ‘ or ‘ Sonos App missing devices’  there are many  with the same issues as I. 

I am delighted you don’t have issues as I am a massive fan of Sonos 

I’ve used Sonos for over 10 years in the same house and until about 6 months ago all was good

However the fault I am seeing is real and I have not changed anything in my house.        Not moved house nor changed Router or indeed phone in 3yrs.

I’m truly sorry you’re having such a bad experience. What did Sonos support say when you submitted a diagnostic, and called them?

I, too am a long time user of Sonos, with over 20 devices in my ‘home’, and a couple of dozen more spread across two other ‘homes’ that I support. 

Based on what you’ve posted, since I don’t have access to any hard data like Sonos would, I would diagnose network issues, most likely dueling IP addresses, which would cause all of the issues you’ve put in your post, but not issues caused by the app. This is a semi-common occurrence due to the fact that when there is a Sonos software update, the devices reboot, and ask for a new IP from the router. If the router has ‘forgotten’ (Power surge? Some other failure?) what it has handed out, it can easily give IP addresses that are already in use. 

As a test, I’d start with a network refresh, by unplugging all Sonos devices from power, then rebooting your router. Once the router has had a couple of minutes to come back up, I’d power back on the Sonos devices.

But ultimately, this is a guess, based merely on my experience in helping others on this forum. Your best bet remains to contact Sonos support directly, and provide them hard data which will help them figure out why your experience is so troubled. 

I have always had exactly the same issues with Sonos on my iPhone yet the app always works fine on iPad!!

What did they say, when you called Sonos Support directly to discuss it?

The app is trash go back to old one