The Gateway eero acts as your network's router. It must be plugged into your modem via Ethernet at all times. eeros that don’t have Ethernet ports like the eero Beacon and the eero 6 Extender can’t be used as your network’s Gateway eero. Echo devices that support eero Built-in also can’t be used as Gateway eeros.
Good news: the eero Beacon, eero 6 Extender, and Echos with eero Built-in can be used to wirelessly extend your existing eero networks. Adding another eero can help cover wifi dead spots. See this article for steps to add more eeros to your network.