Once you’ve ‘registered’ it successfully with Ethernet, unplug the Ethernet cable. It should then connect to your mesh. (I use TP-Link Deco mesh here for Sonos.)

Not sure what you mean by register though. 

It doesn’t connect to my mesh, as I can tell it’s connected to SonosNet as my TPLINK router says the Play 5 is connected via Ethernet …. When it’s clearly running wireless in my backyard.  
 

What I mean by register is: when I hard reset the Play 5 and try and set it up as a new device outside for the first time into my Sonos System.   

I’m just not sure on this... I wonder if any of the other learned forum users can advise?

(By the way, don’t mark your post as the answer otherwise no-one will know the issue remains unsolved/outstanding, and they will pass over it.)

As long as your Play 5 sees a wired Sonos device it will try to connect to Sonosnet.

You can work around that by disabling the radio in the wired device but that also disables sub/surround connections.

Usually best to just pull the Ethernet cable.

I got the similar issue in my flat which is full of concrete and metal and a lot on nearby wifis. I am aware of this so I am using two APs at the opposite points of the flat.

I connected Beam with wifi cable to be sure that it has the good connection. This was a bad thing to do as then Play:5 in my kitchen starting to connect to it via sonosnet and not to the AP next to it.

In my case the Play:5 was switching constantly from WM:0 to WM:1 (connecting to my wifi and sonosnet back and forth). And the Apple Play lossless was stopping. Funny there was no issue with YTM.

My advice for now - do not connect any cables to Sonos speakers if you don’t know how it works as the speakers with cable has priority and all the other speakers will try to connect to it even from far far away.

The cable connection is broken by design in Sonos and I believe it needs to be fixed while rethinking how it should really work in the whole building not just in single room and maybe using some better automatic algorithms in the firmware. Even a way to disable this SonosNet priority over wifi “feature” for given speakers would solve all the issues for people with existing good networking setup.

Sonosnet does not work very well when you place speakers on the very borders of where it reaches and where better wifi is available. Then it will switch between the two as you experienced. I would not call it "broken” because of that, but is is a disadvantage of the Sonosnet solution. Since Sonos seems to step away from Sonosnet (it does not work on the newer speakers and Sonos has discontinued the Boost) I do not think Sonos will fix this with solutions like better networking algorithms.

I meant it’s broken in terms of is it working or not for regular user.

If the user does not use any cables and has good wifi network everywhere it’s working. If he plugs a cable to one speaker it stops working, the whole system stops playing and a lot of frustration comes and time is spend to diagnose it. Just a single cable which should make things better not worse.

I know it’s working ok in the same room between home theater speakers so it should be used in this case only. I cannot think about what is the use case of this SonosNet priority, it should just use the strongest wifi signal, whether it’s sonosnet or not.

If you want some Sonos to be hardwired and others to use your own WiFi (not SonosNet), once you plug the Ethernet cable into the speaker, disable the WiFi interface for that same speaker. That will prevent that particular speaker to start broadcasting SonosNet and the other speakers from trying to join the now-broadcasted SonosNet network.

Once that done, you now have Sonos on both your own WiFi and Ethernet, not using SonosNet.

https://support.sonos.com/en/article/disable-or-enable-wi-fi-on-your-sonos-products

Just note that the "disable WiFi" does not disable your WiFi as you would think, it disables the whole radio making the Sonos 5gHz Sub/Surround unavailable too.

So the option is usable for standalone Sonos but not for home theater sets.