Mesh networks cause a lot of problems because they sometimes use different subnets and regularly change channels. The only recommended approach with a Mesh network is to connect one of your Sonos devices to your router via an ethernet cable, remove your WiFi credentials from the Sonos App and then use Sonos net to connect your devices together.

The easiest plan would be to wire one or more of the SONOS players to your network, make sure that your AT&T gateway is not in Bridge mode and set your Eero to Bridge mode.

I recommend that you remove any WiFi SSID’s defined in the SONOS players -- unless you are using MOVE or ROAM. Do this while one or more players are wired to the network.

I’m not familiar with the AT&T Gateway. It would be best if you can turn OFF its WiFi. If you cannot turn it OFF, make sure that it uses an SSID that is unknown to the SONOS system and any devices that have the SONOS controller installed.

Avoid any SONOS factory resets without further consult.

Thanks for the advice. My router is in an inconvenient part of the house for hardwiring to a Sonos speaker. Any other ideas? I guess I’m out of luck for Sonos working with my mesh network? 

For those of us with a router too far from a speaker to wire it conveniently, Sonos makes a inexpensive device called a BOOST. I happen to use one, it keeps all my Sonos devices ‘wired’ and on the same subnet, even though technically, only the BOOST is wired. 

If you have no need for a speaker at the router main hub, the Sonos Boost is a lower cost wired unit alternative.

It’s not my favorite setup, but you can wire to an Eero after they are setup.

BOOST is a better idea.