Please answer the questions below before we proceed.

• Is that the only speaker connected via ethernet?
• Do you have other speakers connected via Wi-Fi?
• Is there a Sonos Boost in your setup?
• When you remove the Ethernet cable did you re-enable WiFi in the app for the Play 3?

Contact Sonos support. 

That is the only Play 3 speaker connected to modem via intranet cable, when it connects the other majically works and connects to the system without intranet cable, when I unplug both don’t work..

and yes we have a Sonos Beam & Sin for my TV which works well, as I said my Play 3 are 10 years old but work fine, but need to be connected via cable, and I have reset both Play 3

Without answers to ​@AjTrek1 ‘s questions, it’s hard to give advice. If you’ve changed networks, then you probably should read the new router - installation instructions FAQ, which also applies to ‘new’ SSIDs. 

Not quite sure what ‘reset’ really means in this context.

I’m assuming when you say ‘Beam & Sin’, you mean ‘Beam & Sub’. If so, it may be just as simple as performing a factory reset on both the PLAY:3s, and then adding them back to your existing Beam & Sub.

For what it is worth, my PLAY:3s continue to work fine, I was listening to them today, using S2 and iHeart Radio, on my WiFi signal. 

I think it has something to do with 2.5 gHz on the Telstra Modem Gen 3 which uses 5 gHz, which is not compatible with the Play 3, according to Telstra, I would have thought this was a common issue mixing new and old Sonos systems with new Modems, as I said they work when wired to the modem.

From what you’ve said, I suspect that when you connect a Play:3 by Ethernet cable, SonosNet is kicking in and creating a link to the other one. 
 

Does your new router only offer 5Ghz wifi? I’m not sure Play:3 supports that. 

It’s only the Telstra Modem gen 3 it offers both 2.5 and 5, I wouldn’t thought the Play 3 could see this, it unusual b/c I have the exact set up at home with old Play 1 and no issue, I think it might be just the Play 3, it’s a bit of a flaw in the design, I’m surprised it’s not a common issue with all Sonos users,  Telstra think I need to go into the Modem and unclick one of the boxes.

thus is what Chat GPT advises:

• 2.4 GHz vs 5 GHz: Play:3 only supports 2.4 GHz Wi-Fi. Make sure your router is broadcasting that band.
• Channel: Sonos prefers channels 1, 6, or 11 — check your router’s Wi-Fi settings.

I don’t know why it would be so picky

The Play:3 can connect to both 2.4GHz and 5GHz.

See the following support page for supported WiFi modes and security standards.

https://support.sonos.com/en-gb/article/supported-wifi-modes-and-security-standards-for-sonos-products

Is the Telstra modem new?

In the Sonos app, under Settings > Network > Manage Networks does the WiFi network name appear?

These Play 3’ are over 10 years old, idk if they have been designed for both, all my other Sonos systems are connected, so the WiFi is coming up no issue, I just can’t add the ole Play 3s

The link I provided confirms they work on both 2.4GHz and 5GHz so your router’s WiFi security settings are likely the issue here.

Edit: If your router is set to WPA3 then the Play:3 will be unable to connect as they do not support that level of security.

I think you are right something to do with the modem settings 

Most new gear has backward compatibility options available, sometimes well buried and hard to find.

Sometimes new stuff, like Airtime Fairness is glitchy and needs turned off when connecting older hardware to new kit.

Yes, like I said, it’s a techy issue, idk why the Sonos Play 3 (ole gen) works with one modem and not the exact other modem, it is picky, & frustrating, I would have thought others had a qik fix..other than turn off and on the modem..it would be good if Sonos had a physical reset button, rather than push down two buttons it’s so archaic IMHO