The factory reset likely removed all the easy options, support is likely your best bet now.

You could try adding static/reserved IP addresses for all your Sonos, then powering them down and rebooting your router. Power the Sonos back up.

Connect the problem One via Ethernet and power-cycle it if it still doesn’t connect.

Thanks for the response I'll look at that when I get chance.

I assumed a factory reset would return the speaker to the state it was in when I bought it & it was easy to add to the system then, but who knows.

Weird update this morning, both 'ones' are visible in the app, but the beam has vanished & I haven't touched the settings on that ‍♂️

That really does sound like duplicate IPs, as @Stanley_4 suggested. Or maybe wifi interference .

It's very odd, there's nothing changed WiFi wise for a few years, no new tech anywhere nearby or anything either, I was hoping it was the app glitching out and an update would fix it, but I guess there'd be a lot of similar complaints if that was the case.

Update on the (re) adding the ones as surrounds issue, I thought it was the one speaker that had the issue, but actually it's whichever speaker I try to add second, the first one, whichever I try, plays the chime & I have to click which side the sound is coming from, then when I move on to the second speaker, it can't be found ‍♂️

‘Nothing changed’ in Wi-Fi is concerning. There is always something changing in Wi-Fi, from router firmware updates to outside sources possibly causing interference. You may not have changed anything, sure, but that doesn’t mean nothing has changed, so there is some cause for investigation. 

 If I were you, I’d be submitting a system diagnostic within 10 minutes of experiencing this problem, and calling Sonos Support to discuss it.

Sorry I meant nothing has changed as in it's not a new router or a new set up.

I have now spoken to them on the live chat, sent two diagnostics but we're no further forward, to be honest i'm not sure they had a clue but the phone line isn't open til tomorrow so I'll have to try then

Meanwhile, perhaps try removing the surrounds from the Beam via the App and then powering everything off, including the router and bring all back online in this order…

Router, Beam, each surround.

Then see if it will allow you to bond the two speakers back as surrounds and hopefully it will resolve the matter for you.