The other way this could be done is if TLS-SRP was more widely supported. This variant of TLS doesn't depend on certificates at all and instead uses mutual know

The other way this could be done is if TLS-SRP was more widely supported. This variant of TLS doesn't depend on certificates at all and instead uses mutual knowledge of a password to authenticate the connection in both directions. In the case of a router this would be written on a sticker on the device for initial configuration.