Wish you hadn’t done the Factory Reset, that just makes things harder.

Try connecting to your router with Ethernet.

Since you wiped everything with the reset go back and follow the install steps for a new system.

When I connect to the router with Ethernet it keeps asking to disconnect when it tries the update. Am I doing something wrong there?

Every step so far I am doing the Add Speaker or component option, not join an existing system (which cannot find one on my wifi).

Edit: just tried to plug it back in again and as soon as I ask to add speaker it asks me to remove the Ehernet cable.

Using an iPad vs iPhone v2 app made all the difference (worked first time).

My experencies

Tried to install a Play 1 in the new Sonos app. At first connect it already said there was an update needed for the Play 1 so it kinda updated it but the never completes.
Retrying or resetting didn't work.

What worked for me is:
1. Power down de Play 1 and plug in an ethernet cable
2. Factory reset the Play 1 while keeping the ethernet cable connected.
3. Go drink some coffee or tea and wait until it is resetted and blinks green.
4. Now install it again while keeping the ethernet cable connected.

Had three S1 Play:1 with this issue that succeeded with this workaround, hopefully it works for you too.

Using the iPad app worked straight away? I might steal one of the kids iPads and give that a go if that’s an option.

@Ed70 thanks for that advice. I’ll give that a go tonight as everything else I’m trying is not working.

Oh you legend. Used the kids iPad. Got a 30 error on the first try. Then straight through on the second. Cannot thank you enough.

It finally worked for me on a Play 1 I had been attempting to transfer from a different system. I tried the iPad a handful of times over the weekend with no luck. I thought the latest app upgrade for the phone would solve it but no luck with that last night either. I was starting to think maybe it was a hardware issue. This morning I saw your message, grabbed my iPad, downloaded the latest app update on the iPad and it worked on the second try for me as well. Only 20 attempts total! Thankfully the rest of my system worked on the first attempt.

New Eero Mesh with a new network name. three Play 1 speakers and one Play 3. After 2 days of frustration, trying everything including dedicated DHCP, Ethernet updating, Sonos tech help (useless), factory reset on the speakers-- just everything -- I found this link. Thank you! It should be one of the first things Sonos should have you try. Use your iPad. Bingo. Had it all up and running in 30 minutes. Thank you, thank you, thank you.

Glad people are finding the solution too. When I went looking there was nothing. Brad is the hero here.

iPad fix didn’t work for me, and when I tried this, it asks me to unplug the Ethernet I. Order to install the update. Did it ask you that? 

Can you say your process? Did you start from a factory reset all over again when using the iPad method? Also how long did the update take when it worked? 

I kept getting the Ethernet message, so I factory reset. Then used the iPad and got the 30 error. Next try it clearly was working after 30 seconds.

Still took me about 10 minutes to run through the whole setup process but it looked like it was working the whole way through.

I have 3 play ones and 2 sonos ONEs. My sonos ones are connected to the updated app. I cannot get my Play Ones to update with the new app. I have factory reset, tried with an ipad instead of my phone, tried to connect via ethernet cable but sonos wont allow it. Everytime I try to update I get 50% or maybe 75% before it times out and my playones start flashing red. Ive tried to update at least 2 dozen times. Would downgrading to s1 remedy this? 

I'm so so happy for this comment!! I used to use my Play 1 on a daily basis but for the past year have not been able to get past the update error. I used my kids iPad to push the update through and am back up and running!!! This suggestion works!!