I don’t quite understand what you mean by ‘software outdated’ - unless you mean the C:A is only capable of running on S1 and your present Sonos System is S2?
Either way, you cannot make any use of a Sonos Ethernet port for networking purposes unless it’s connected over a SonosNet wired/wireless connection… and in any event, there’s no way through the Sonos App itself to output Sonos audio to a device that is not held in the same Sonos Household and particularly via an Ethernet connection to your old C:A.
You would likely be best to setup the C:A to run (wireless/wired) standalone on your Sonos account using the (separate) S1 App, which will happily co-exist with S2 on your compatible mobile controller device that meets the current requirements shown in this LINK