If you just want the green lock to show up, the device can get a certificate from Let's Encrypt. The manufacturer would need to provide an API that lets the device pass the DNS challenge.

For example, you could have serialnumber.manufactuerer-homedevices.net, and each device would get a cert for its serial's host name. Ideally, you should properly secure that API with some form of attestation key included on the device. Alternatively, the host name could be e.g. the hash of the devices' generated key (that way you could ship the devices without placing individual keys on them, but the host name would change after a factory reset).

Making this actually secure is hard, though, because you need the user to visit the URL for his device. If an attacker can simply get a cert for differentserial.manufacturer-homedevices.net and direct the victim there, you don't win much actual security.