“The SonosNet Channel setting is available while you have at least one Sonos product wired to your network with an Ethernet cable. If all your Sonos products connect to your Wi-Fi network, the SonosNet Channel will appear grayed-out in the Sonos app and cannot be changed.”

https://support.sonos.com/en-us/article/change-your-sonos-system-s-wireless-channel

FYI… when choosing which speaker to wire to the router, it is usually best to avoid choosing the Sub or surround speaker. Try wiring another speaker that is not a subwoofer or surround speaker.

It is not possible to wire the play bar to the router because the connection cannot be reached as it is on the wall. The set up is play bar, rear speakers and sub. 

With only those four speakers, it’s better to have the system hanging off your WiFi network, if you’re unable to wire the PLAYBAR directly on a permanent basis. Temporarily wire the PLAYBAR and then go in to Network Settings in the controller, and add your SSID and password to the Sonos system that way, then you can remove that temporary connection via Ethernet cable. 
 

If you’re unable to wire the PLAYBAR at all, remove the surrounds using the controller, and set them up as their own ‘rooms’ in the Sonos software. Then wire one of those two speakers to your router, and make the changes, including the SonosNet channel, to your network settings. Remove the temporary Ethernet cable. Finally, add the two surround speakers back to your PLAYBAR. You’ll have changed everything to hang off WiFi at that point, and as discussed above, the SonosNet channel won’t make any difference, since once you remove the Ethernet cable, it will no longer be in use. 

Since none of you devices seem to be wired, how can you have Sonosnet?

You could try the Sonos Boost to start Sonosnet though.