DIAGNOSIS The eero network is active but the gateway eero has lost its upstream WAN connection. This is a modem or ISP issue.
FIX Check the modem. Is its online/DSL light solid? Power cycle the modem: unplug for 60 seconds. Restart the gateway eero after the modem is fully online. In the eero app, go to Settings > Network Settings > DHCP & NAT. Ensure it's set to 'Automatic'.
IF THAT DIDN'T WORK ISP may require PPPoE or VLAN tagging. In the eero app, go to Settings > Network Settings > ISP Settings. If your ISP uses PPPoE, enter credentials here. For VLAN tagging (common with AT&T Fiber), set VLAN ID to 0 or as specified by ISP.
HEADS UP A double-NAT scenario from a prior router can cause this. Connect the eero directly to the ISP modem, not another router. If you must use a prior router, put it in bridge mode first.