FWIW, most people only have one TEG (Tesla energy gateway), although its not unheard of for someone to have 2 if they have large electrical service (above 200amp). I have never read anything at all about a "serial" connection in a TEG. The wifi connection on a TEG is used for the TEG to connect to the internet (not to communicate with any other devices connected specifically to it, like powerwalls.).
You can connect a TEG via the ethernet hard wired connection, to connect it to the internet. The connection between powerwall and TEG is a specific one via some cable / communication I am unaware of, but its not wifi.
Ok let me expand, I believe the TEG communicates from the current clamp sending unit which has a small aerial on it to the Tesla powerwall which has what is labelled as a gateway connected to it. I don’t want any WiFi in my home for health reasons therefore I want to be able to connect the current clamp to the TEG without the use of WiFi. The gateway communicates via an Ethernet connection to the internet so I can monitor the battery via the app. I hope this is a little clearer.