DIAGNOSIS The new eero is not in a discoverable state, or your phone's Bluetooth is interfering with the setup process.
FIX Follow the precise power sequence. Plug the new node into power near your gateway eero. Wait for it to pulse blue. On your phone, disable Bluetooth. Open the eero app. Tap Add or Replace eero Device. Follow prompts. The app uses a local wireless broadcast, not Bluetooth, for discovery.
IF THAT DIDN'T WORK The eero may be on an old, incompatible firmware. Hard reset the new node (hold reset for 7 seconds until yellow flash). During setup in the app, when prompted to scan the QR code, manually enter the 6-character serial number from the bottom of the unit.
HEADS UP The eero 6+ must be within 10 feet of your phone and the gateway eero during setup. The system downloads configuration from the cloud. A slow internet connection during this phase can cause the app to timeout.