What should I do if HomeKit says "Unable to Add Accessory"?

I cannot add any new "Wemo smart switches" to the default home in the HomeKit. I can create a new home, and they will add to it, but that is not helpful as that new home is not seen by the hub(s). I have disabled 5g in my network; I have all the updated applications and followed all of the standard setup protocols. When using the default home in the HomeKit app, I face the following error message after scanning the device's QR code when attempting to add: "Unable to Add Accessory." To add this accessory to your home, connect your iPhone to your HomeKit enabled router, then try pairing. If paring continues to fail, reset your router." I've reset everything. I worry that it's a function of my Eero mesh network.

[Re-Titled by Moderator]