@RoodAwakening

Here is what Sonos give us as official information. 
https://support.sonos.com/en-us/article/surround-sound-guidelines-and-limitations

So if you have lan cabled ALL of the home theater devices, imo that should work. Even there isn’t said anything to deactivate Arcs wifi module, imo in case of all devices cabled you could do so. 
If that won’t work for you,  I‘m pretty sure you will need the original Sonos adapter. 

Thanks, Ralf. I saw that “Limitations” advisory a couple of weeks ago but it doesn’t reference how to set it up if your home theater speaker--which, in my case, is the Sonos Arc soundbar--is set to Ethernet. One would assume that the app would recognize how the primary speaker is configured and would connect accordingly, at least as long as the infrastructure is set up accordingly (i.e., it would default to Ethernet if all speakers were connected via Ethernet). Unfortunately, my wired Era 300s are stuck on WiFi (with no way to override it via the app ) while my Sub and Arc are on Ethernet, meaning I get no surround channel. 

If you already have the Arc hardwired, you really don't need to hardwire the 300s. Make sure that WIFI is enabled on the Arc, Some will say this is not needed but it is what I have done. I have no connection issues. Then if the 300s are already connected to your homes wifi, go to add surrounds in the room settings. you should see the 300s there to be added to the Arc.

I just recently purchased a pair of Era 100s for surrounds and the setup was easy. My Arc is hardwired with WIFI on , bur my Sub and surrounds are wirelessly paired to the Arc. Do you have a managed switch in your network?

AIUI, Sonos say that you can Ethernet connect the Eras if the Arc is also wired. So, to me, I’d think you need to disable wifi on all of those speakers in order to force the wired connection. I don’t know, but it seems logical that you might need to disable wifi on the Eras before adding them as surrounds. 

@RoodAwakening 

Seems to me you will need the Sonos original ethernet adapter. Maybe you can check your current adapter by sending a diagnostic and Sonos support can see if the adapter is working. 

Technically no, but usb-c to Ethernet adapters do require a driver. Many adapter brands use the same chip with a different id so a single driver works with multiple adapters as long as it can match the id in the driver.

The firmware on the Sonos devices will have some drivers in it, but it is unlikely to have all the drivers a general operating system has because it is for a specific hardware system with limited resources compared to say a pc.

It is no different to why TVs, for example, only work with some brands of usb to Ethernet driver, or the Steamdeck operating system only works with certain hardware.

If a 3rd party adapter happens to use the same chip as the Sonos branded one and has supported ids then it is likely to work. Random usb-c to Ethernet adapters aren’t unless you get lucky with driver support.

Presumably, if you configure the Era 300’s as standard speakers, not surrounds, you can turn off the WiFi in the app, and if the generic adapters work, and allow them to play music, the drivers must be working? 

@Ian_S If that option is available, then indeed the adapter would be compatible with the drivers within the Era 300s. I expect that there will be some that just work, the trick is finding them.

In the opening post it was stated that the 300s wouldn’t connect to Ethernet at all using the adapters and later there is no option to disable WiFi , so it sounded more like the adapters aren’t working to me.